I played Arc Warden as support in a few unranked games and three ranked. I won all three ranked games. I consider it very viable and even strong. It would take a lot more games to know for sure. I'm writing this guide to share my thoughts and get more people to give it a try.
It's comparable to Venomancer. Flux, similar to Gale, provides a strong slow and damage. Spark Wraiths, like Plague Wards, give long-range vision and slows - perfect for chasing, preventing jukes, canceling blinks, safely pushing towers etc.
Compare magnetic field to Ogre's bloodlust - attack speed steroids can be very strong on supports.
Zoning and killing the offlaner.
Awesome early game roamer. (They can't run and can't hide)
Low mana costs, plus "free" spells when you use the clone.
Strength and intelligence gains are adequate.
Pushing power / area control. Wraiths on the chokepoints make it hard for the enemy to approach unseen. Magnetic Fields help to quickly take towers.
Insane single target nuking. At level 11, you can double flux, double wraith a target to deal 1,600 magic damage in 6 seconds for only 155 mana! (60 second CD). If that's not enough, throw in two more wraiths for 2,160 damage for 235 mana. That's not counting the MF-enabled right clicks.
Ability to cast spells usefully from a safe range. Wraith has mega range, MF can be cast on allies from behind, and if necessary you can use the clone to cast flux.
You can break linken's with Flux, then cast it again from your clone.
Melee heroes who want to blink in and chase down or run out (axe, slardar, clockwork, ursa). They will get kited and not be able to run away. Also, wraiths will make it hard for them to find the approach to blink.
Magnetic Field will counter ranged carries until they build MKB.
Flux's mutability means the opponent can attack in numbers and beat you. (Hopefully someone on your team has aoe which can punish that.)
No hard cc.
Flux can be dispelled, MF countered by MKB.
Illusion heroes or chen creeps who can overpower your single-target spells with numbers.
Mobile heroes like anti-mage, weaver, ember spirit and morphling.
Someone out there is thinking -- Arc support, MUST buy ARCANE'S! You could have DOUBLE arcane boots every 60 seconds OMG Kreygasm!
NO! Just no. The same price you can have tranquil boots plus eight clarities, addressing your low armor and slow movement problems, keeping you healthy and in the field instead of running to base. As you see in the next section, all the other items you want to buy will give you mana anayway. By the time you get them, arcane boots will be a total waste. You do want mana regen so you can keep scouting with your Spark Wraiths, but you don't have high-cost ultimate or other spells so your total mana pool ins't really the issue. So just get clarities until your levels and items solve the problem. And early game, you can't spam wraiths, use them intelligently.
Someone else is saying: You MUST buy Hand of Midas OMG dolla dolla dolla. Stop being such a greedy *****! You pick a position 5 hero for his utility WITHOUT items so that your other teammates get to farm. When exactly are you going to save up the money for a HoM after buying couriers, wards, tp's, sentries, smokes, dust etc? Yes, some supports do this anyway. AA or Venomancer might take a timeout from fighting to farm it -- well they want to get agh's + level 3 ults (XP from HoM is important) so they can do massive aoe damage. When Arc Warden gets his level 18 -- the enemy will be quaking in their boots at the thought of his clone being on a 40 second CD instead of 50.. ya right. And there is no one item which will massively upgrade Arc's power. Plus, you want to save your ult for fights. You may be hanging around a carry or a tower, waiting to counterinitiate, for a long time and your HoM won't
That said, if the game is stalling out because your team can't breach high ground, and you find yourself with plenty of money and no item you really need, go ahead and get it. I did this in one of my three ranked games, against a sniper, where the pos 4 bought half the wards for me and I didn't need force staff at all. I ended up with BoT's + mjollnir on top of solar crest drums. So it is an opportunistic option but don't base your strategy around it.
Another don't-get item for me is urn of shadows. I tried it and it wasunderwhelming. You don't want to waste your clone just to get more urn charges, and it's wasteful of item slots.
As I mentioned in the last section, tranquil boots and clarities is all you need to stay in the field casting your spells. Add a sage's mask when you have the money. Even if you have a jungler and have to buy EVERY DAMNED WARD against a 5-invis team, this very cheap build will keep you going, controlling an area, defending towers, helping your team farm.
Medallion is a great first item because it's only 1200 gold and gives you armor and mana. The active is awesome to use twice in fights and helps your teammates kill someone damaged and slowed by your skills. If you bubble defensively, you can boost an ally's armor when he leaves the bubble.
Of course force staff is awesome on all supports. Some games, you really need a force staff first. Against riki, clockwork, etc. So get it first if you need it. Other games, you don't need it at all. Use your brain don't always follow the guide.
Force Staff synergizes with your chasing potential, so you can force yourself into range for a Flux. It also gives you an option to stay safe somewhere, and force your clone in to cast spells. Maybe you can use it on enemies who are muting Flux or who run into your bubble.
Drum's is a nice luxury item. As long as you don't use your last charge, you can cast it every time you use your clone. Drum's active in every fight all game is a great value!
Upgrading medallion to solar crest is always good. Even if opp's build MKB to counter your bubble, the extra armor, recently buffed to 12, still helps.
If you get to medallion, force, drums, you will have enough mana to spam Spark Wraith's nonstop.
I rushed three points in Flux because I wanted 40% slow. 50% is overkill.
If you want to max your damage instead, get Spark Wraiths since the low CD means you can recast it for more damage.
When to get the value point in Magnetic Field is situational. In a pushing lineup, perhaps with a luna or drow, I would consider maxing it earlier.
Thanks for reading. Try it out! This is a nice way to play an interesting hero without being anti-fun cancer (split push and farm for 50 minutes).