Hoo boy. Well, here goes!
Such a great unit on paper, and yet so lousy on the table. At 50 points per model this unit is overpriced or underpowered, depending on how you look at it. A full unit of 5 with an upgraded exarch will cost you just over 250 points with Skilled Rider and Withdraw. In truth, I've had better luck running a jetbike autarch and spending the extra 100 points on unit upgrades.
Without getting into everything that's wrong with this unit (which is an article in itself), I would propose the following:
If you were to keep the unit at the same point cost, make the Laser Lance a rending weapon, all the time, somewhat like the harlequin's kiss. Another option would be to make the Laser Lance the equivalent of a krak or haywire grenade.
Otherwise, reduce the point cost by 10 points per model. Suddenly you can have your fully kitted out squad for under 200 points, which isn't quite as steep. If you reduce the cost, perhaps adjust the upper limit of the squad to 6 models.
This is another problem child. You pretty much have to decide whether to dedicate the unit to blowing up guardsmen or popping tanks, with no real middle ground allowed. Their speed and the potential to redeploy and thus bombard enemies is cool, though I'm uncertain how effective it is against the ever-present Marines. Overall I believe a slight point reduction is in order, maybe dropping the models to 19 or 20 points a piece.
Flying coffins bristling with guns! I love these things. I don't have a problem with anything about them. If I were to make one change I might increase their squadron capacity to 5 rather than 3, or perhaps allow one to opt out of a primary weapon in exchange for a small transport capacity like the land speeder storm.
These are my favorite and default Fast Attack choice. Again a slight point reduction may be in order, but overall I think their ability to jump twice in a round, their 3+ armor save, and the large number of shots they can put out, combined with their Exarch options, make them an incredibly useful and versatile unit. Pop a properly kitted up Autarch in there with them, and you have the 2nd best hammer unit in the Eldar codex (after the jetbike seer council), and best of all, you don't have to make psychic tests or hood rolls to use their abilities.
One thing I would like to see - and this is a "Nice To Have" rather than "Must Have" feature - is an exarch weapon that somewhat mimics the profile of the Doomweaver.