Quite a lot of DotA players claim that Sniper as a hero sucks. Others, likely (lower MMR) players, say he's OP. I beg to differ with both. He is not a bad hero, so far as I can tell. Quite a lot of people simply build him terribly. They follow guides that have pretty good justification for their claims, but don't really make much sense to me in practice. Thus, given that Sniper is largely item-dependent, he ends up dead.
He also isn't overpowered. He's squishy, and the only way he can survive is if his team tanks while he rips organs out of his enemies from that beautiful 900 range. Now he isn't just squishy. He's pure squish; 3 or 4 hits from a decently farmed (anyone) will absolutely destroy Sniper. He needs good mobility, positioning, and escapes. Another thing guides tend to neglect, but I will most definitely address.
The Game Begins
This is a crucial time for Sniper. This is where he last-hits like a bawss and hopefully is able to harass a kill or two. You'll want to pick up some Tangos for regen, Blades of Attack for easier last hitting, and an Iron Branch for teh GG's.
( Slippers of Agility provide 3 damage and 3 attack speed. 3 of them, which would equate to a Blade of Attack's damage, would cost you just as much, without the ability to upgrade to something useful)
Sidenote: If you're getting harassed a lot, go ahead and pick up a Bottle. If it's really bad, get Power Treads instead of Phase Boots. The extra HP will help you survive the laning stage. If there's a Bloodseeker, you may want to get Linken's Sphere, but for all of these, only if you need to.
You'll want to upgrade those Blades of Attack into Phase Boots as soon as possible (last hitting is more important right now. Don't bring up Power Treads please).
Right now, your only escape is the extra speed from Phase Boots that'll put you faster than most heroes with basic boots. You'll want to pick up a Shadow Blade for extra attack speed and damage(nullifies the use of Power Treads completely), and a handy escape mechanism that will save you early-mid game. By about ~20-25mins you should be able to pick up a Maelstrom for extra attack speed and damage, plus a passive that adds even more damage to 25% your shots.
This is where a lot of freedom comes along. Mid-late game Sniper is an incredibly variable build. It depends a lot on what you're up against. If the enemy is squishy, please don't bother with a Desolator. Just, please. It's such a boring item. Go instead for an Eye of Skadi, as the slow will make escape difficult and the extra stats will provide a noticeable boost to your HP and Mana. Aside from that, for the best attack speed, grab a Mjollnir. Bored of Mjollnir? Get an Assault Cuirass. But be sure you can get the gold for it.
If you want to build into a Daedalus, which is common for a reason, go ahead and grab a Crystalys. If there's a Phantom Assassin in the match, do not rely on your teammates to get a Monkey King Bar. Just do it yourself.
This Game is So Long
At this point, buy whatever the hell you need. You ought to be DPS'ing like crazy going on GODLIKE rampages 24/7 and Sniping the faces off of every person in the history of histories. ENOUGH SAID.
Tango: Self explanatory. This the core-est of item on even the least core-iest heroes.
Iron Branch: GG. 'Nuff said.
Blades of Attack: If you are dead-set on getting Power Treads, go for the Belt of Strength instead. Otherwise, this is a good pickup for last-hitting.
Phase Boots: Use these as a secondary mobility/escape mechanism. They're unreliable mostly with only a 4 second speed boost, so they're primarily beneficial early game when you make them. The damage, though, is notable all up to mid-game.
Power Treads: As I said before, try to only pick these up if you know you're going up against a harass-heavy or gank-reliant mid hero. If you desperately need the extra HP, go with Power Treads instead.
Mask of Madness: I don't really recommend this simply because it makes you even squisher than you were before. It's like a self-inflicted Orchid Malevolence. Just a bad pick for Sniper in my opinion.
Shadow Blade: The Shadow Blade. Apparently, a purely initiative item. Yeah, the invisibility is great and all. But the attack speed it provides to the weak early-game Sniper plus the massive damage boost coupled with Phase Boots simply rivals anything I've found to be effective.
Blink Dagger: I have not played a great many games with Blink Dagger on Sniper, but thus far it's... less than promising. Getting in a great, impervious position using the Blink Dagger is very difficult, if not impossible in some situations. It leaves him with poor attack speed, poor damage, and the need for some sort of escape that isn't disabled by attacks. This is simply my experience. (Note: This could be due to my skill, but I don't quite think that is the case.
Maelstrom: I would like to think of this item as always core on Sniper. It provides attack speed, great damage, and silly passive that gives sweet damage.
Mjollnir: This items offer even more teamfight potential from Sniper, causing the passive to bounce a maximum of 8 times, rather than 4. Not only that, but the active offers a static shield that has a 20% to shock the enemy for 200 damage. Coupled with the 80 attack speed, I'd also say this is at least moderately core.
Assault Cuirass: I'll go ahead and address this here. Sometimes whilst playing Sniper I get bored of the typical builds and such; I like to go wild. Sometimes I'll buy an Assault Cuirass, but purely for fun. If you aren't stomping your game, I would not recommend buying this.
Desolator: This item is great against heroes that rack up high armor values towards the mid-game, making them hard-to-kill targets. Meepo is a solid example, as this will shred any one of his copies to smithereens. If you're playing against squishy heroes, I generally don't recommend this simply because it's not necessary. Eye of Skadi or Satanic would be much more appropriate Unique Attack Modifiers in this instance.
Crystalys: A Daedalus is good for just about any lineup, so if you're looking to get one, go for a mid-game Crystalys. The crits will rock your opponent's socks off.
Daedalus: Crit. Crit. Dead.
Butterfly: Now, this item is not something I often pick up on Sniper. I tend to go for pure DPS build, quick in-and-outs that don't depend on me staying around a fight for too long or getting caught up. If you find yourself getting targeted a lot or fancy yourself a Manta Style user, go ahead and get one.
Monkey King Bar: Phantom Assassin? Never miss = headshot, headshot, dead.
Eye of Skadi: This item is fun. The slow helps you chase down targets(you have a rather low move speed) and the stat gains benefit you in several different ways. I love picking this item up, but only do so if the enemy is often getting away by running. Or you're stomping the enemy. Either scenario.
Heart of Tarrasque: HP. Regen. If you can afford it, are having some HP troubles, get it.
Satanic: Don't need too much HP, but could use extra survivability? Go ahead and go for this. The active ability gives you 175% lifesteal for 3.5s, which could very well save your life multiple times.
Linken's Sphere: Get this if you are continually getting killed by long-range disables like Bloodseeker's ultimate.
Divine Rapier: Interested in throwing the game? This is how you do it.
Shrapnel is a skill I really don't favor overall. Level it once for the vision it provides, and past that I don't really see much of a point. It can be situationally useful, but stats benefit Sniper more in my opinion.
Pick up a level of Headshot for that beautiful 40% proc chance. The extra damage can also gain you a last hit or two you might've otherwise missed. After that, level up Take Aim as often as possible, taking Headshot every other level when you cannot, and your ultimate at 6, 11, and 16.
To address Take Aim: Sniper is a squish-master. His saving grace, or his balancing point, is his range. Take advantage of that, and don't ignore his potential. In the midlane with decent lane control, Sniper can literally sit at tower and get last hits. That is incredible. After Headshot & Take Aim are maxed, take Stats whenever possible. They're infinitely more beneficial than Shrapnel. Max that by somewhere around level 25.
As with every guide, I will pressure this. Positioning. Positioning with Sniper is key. If you can get behind a couple of enemies who're pushing a tower while your team comes at them from the front, there'll be no chance they get away. Of course, if someone ever is getting away, be sure to keep your trigger finger on Assassinate for the longshot.
Look for choke points. Look for hidden areas. Become aware of the points on the map which're difficult to discern if there's a hero present. Set up your ganks so you can come in last, keeping you out of their sights for long as possible.
When you're looking to gank in the early-mid game, be sure to look for lanes that are pushed far towards your own side's towers. That'll make it easier to catch stragglers without tower diving.
It's not too terribly difficult to get these bits right, just play smart. Don't walk into engagements you can't come out of. If you see a chance to Assassinate, unless it's guaranteed for another carry, just do it. If you died while in a fight, re-assess that quarrel and see how you might have been able to play better. Some key things to look for are: Did you do all you could in the teamfight? Was there a better location for you to come from? Should you have been there at all?
Re-assessing losses can be an invaluable asset when learning any hero, so try to do it at least every now and again.
Don't jungle. Just.. don't do it. Unless you have nothing else to do and you can tank it without losing HP, just don't.
Creep lane pushing is very easy with Maelstrom, even more so with Mjollnir. Push lanes often around mid-game.
Gank often after you get the first level of your ultimate. You need the level and gold to finish off your Shadow Blade as soon as possible. Other than that, just make sure you have teammates to tank the hits while you shred face from behind-lines.
All-in-all, Sniper is powerful with the correct items. That's... about all I have to say. If I didn't cover anything here, just tell me. I'll gladly edit my guide and cover it. Thank you for taking the time to read over what I have here.
Oh, I heard you have a face? Let me take care of that.