Invoker is widely considered one of the hardest (if not the hardest) heroes to play in the game, and also amongst the most unique (due to the fact he cannot put abilities into stats and the fact he has to Invoke his abilities). He is also considered one of the strongest. If you master him, you will have a LOT of power. Mastering him is the difficult part.
Invoker can fit a LOT of roles. His most viable ones, in my opinion, is as a full on semi-carry or as a semi-carry mixed in with support. You should never really search for a match thinking "Ok, for the next 5 games, I'm going to play Wex Invoker" (for example), as they all work in different situations.
Invoker usually fits into mid lane quite nicely, but due to his myriad of CC and semi-supportive abilities (even abilities like Cold Snap are brilliant for two people), he is also really good in a double lane, with any sort of hero, too.
Other than that, he's an absolute blast to play, especially due to the satisfaction you get by having access to such a large amount of spells, and when you get them properly, saying he's fun is an understatement.
LET'S GET DOWN TO BUSINESS! (We don't need to defeat any Huns today, though)
As with any hero, Invoker has some pros and cons. Invoker is blessed with having a spell for absolutely every situation, but that doesn't make him the best at every single situation. I'll highlight each of the major advantages and shortcomings here.
Good question, oh dear viewer, good question indeed.
As the names of the build imply, Wex is, basically, better for being a CC based semi-carry, and Exort better at being a harder carry (though, never expect Ursa-like damage). You should pick according to what team you have, if you have a few hard carries or lots of damage, I'd opt for a Wex build, but if your team is more CC/support oriented, opt for an Exort build.
Why is Exort a Harder Carry?
Basically, your main damaging abilities are all increased by Exort. Forge Spirit's damage, health, and armour, Sun Strike's damage, Chaos Meteor's damage, Deafening Blast's damage, Alacrity's damage, Ice Wall's damage, and, needless to say, having 3 instances of Exort at rank 7 will give you 63 attack damage (that's more than a Sacred Relic) AND 14 intelligence, giving you a further 14 damage, resulting in a grand total of 77, and a larger mana pool and regeneration.
Why is Wex More Supportive?
Wex does a lot of things for your CC/supportive abilities. Namely, it increases the mana drain from EMP, greatly increases the range on Tornado (giving an enormous 3200 range at rank 7), increases the attack speed on Alacrity, disarm time from Deafening Blast, movement speed on self with Ghost Walk and rolling range of Chaos Meteor. You also have the more mobility around the fight with Wex's movement speed, and the fact are slightly more tanky (+2 armour at rank 7) from the bonus Agility you get from it.
WARNING TO NEWER PLAYERS -
Wex based Invoker is easier to understand and manage to begin with. Exort based Invoker requires better aimed Sun Strikes, better micromanagement of Forge Spirits, your escape mechanism isn't as good, and you are the one relied upon for the majority (or at least a large portion) of your team's damage.
Invoker requires a LOT of practice to get correct. In your first match, I'd be surprised if there wasn't at least one situation where you thought "I'll just Invoke Ghost Walk and be safe from this gank" and end up with Tornado or even Ice Wall. Not to worry, there are remedies to this, you won't be stuck as the worst Invoker in the world forever!
Needless to say, the best way is by playing him. Sometimes his skills make sense as to why they are what they are (such as Chaos Meteor, it's made of fire, so it needs at least some Exort in there, but it's a meteor and as such moves fast, hence, Wex. Quas wouldn't make much sense as when are meteors ever cold?) but some of them, especially very situational spells you tend not to use often (like Ice Wall, which sees little to no use in the laning phase) can just seem to elude you.
However, I got linked this 'trainer' by a friend as I was learning him, and after but a few goes it really helped me learn Invoker.
It's surprisingly fun/addictive, too, believe it or not!
If you're new, you can always click on the question mark next to Invoke and see the list, or have a reference image open such as the one below. At first, you will forget everything. Invoker is naturally hard to learn, and after a while you will, indeed, know them all!
Good question. It does seem odd, at first, that, for example, in the Exort build I end up getting 2 more points in Quas after 4 points in Exort. I'll explain my reasoning behind both builds' skill orders below.
Why THAT Exort Skill Order?
As mentioned before, I take Quas first for quick access to Cold Snap, and Quas gives insane health regeneration, so I took another point in it for early game regeneration. After that, I take four points in Exort, then rather than finishing it off, take two more in Quas. To an outside observer, this may seem silly, some may ask why I didn't get Quas earlier (which, might I add, is completely viable in harass heavy lanes), or why I stopped back after starting to get Exort. The reasoning is relatively simple, you need a bit of attack damage to have the last hit/deny advantage early game, hence why I'd usually not spend too many ranks in Quas early (again, unless the enemy is harass heavy), but I go back for it after 4 points in Exort because once you have FOUR points in BOTH Exort and Quas, Forge Spirit summons TWO spirits. You want this as early as possible - they have ridiculous damage, and shred armour at an even greater rate when there's two.
After that, it might seem odd to you that I opt for taking Quas over Wex. Exort Wex combined builds do exist and do work, though this one is not one of them. The main reason for this choice is the boon Quas grants to Forge Spirits' attack range, mana (allowing for 10 instances of armour melted for each spirit at max Quas rank), and even duration alive. Cold Snap is, of course, great to be improved if you rely a lot on auto attacks, as you do, and the stun from Deafening Blast and slow/duration increase from Ice Wall are also both incredible.
Why THAT Wex Skill Order?
Well, I start with Quas for quick access to Cold Snap at level 2, as it is such an incredible ability it's far too good to miss out on. After that, the other levels in Quas give you such absurd amounts of health regeneration you'll practically never have to go back. At rank 2 of Quas you have more health regeneration than somebody with a Ring of Health. Now that is something. However, getting too many points in Quas is a waste. It loses a lot of its potency outside of the laning phase, and you really start needing to get your damage/abilities really soon. You should take one point in Exort whenever team fights start occurring OR after Wex is maxed, whichever comes first.
After that, we skill Wex first for the fact this is a Wex based, supportive build, and as mentioned in a previous chapter, most of the supportive abilities are enhanced by Wex. After that, I opt for Quas to increase the potency of Ice Wall in both duration and slow, the duration and interval between stuns in Cold Snap, CC duration of Tornado (and damage), stun duration of Deafening Blast, and AoE slow of Ghost Walk.
All of Invoker's skills are listed below on this image (credit to lonjaxon, the creator of this image according to its watermark) -
I hate using the word 'main' to describe Invoker's abilities. Not only is each skill as important as the next (with the one exception in this guide being Forge Spirit for Wex based Invoker, usually that's a waste of time). However, some should be used more often than others on Invoker, there's no question in that.
The Wex Build Main Combo -
These spells will be used either in succession in a team fight and will be your main two spells throughout the game when you go Wex based.
8/5/12 - Published.
8/5/12 - Fixed some typos.
9/5/12 - Added Eul's Scepter of Divinity for Wex based Invoker, and a description in the item section.
16/5/12 - Changed Drum of Endurance to situational, changed Exort starting items to same as on Wex.
25/5/12 - The odd one or two improvements and swapped the order or Exort and Wex based Invoker.
6/7/12 - Fixed the fact they did not have skill builds - they magically disappeared or something, they WERE THERE!
10/7/12 - Added Blink Dagger to situational and did a write up for it.
11/10/12 - Added Force Staff (long overdue) and did one or two touch ups.