I had an experience in a recent game that I thought would help out my fellow Eldar players. I have loved Warp Spiders ever since their heavy flamer template/roll against Initiative instead of Toughness days. I still try to use them as often as I can as their abilities are the signature style of the Eldar.
Anyway, the mission was Capture and Control, a difficult mission to win and an easy mission to tie. In this case, it was turn 5 and we each were holding our objectives. My opponent had been running his Marines to his objective, and as such were strung out. I simply assaulted the far end of his line and watched them all respond to the charge, pulling them 6" off the objective. The game ended and I won.
Warp Spiders are a fantastic harassment unit. They aren't as fast or tough as jetbikes, but they pack a much stronger punch (their guns can punch through most vehicle rear armor, and they're maneuverable enough to get back there). Since they are Aspect Warriors, they also fare slightly better than guardians at close combat, even though they aren't built for it. Furthermore, they can Hit and Run to get out of sticky situations.
Now that they get Deep Strike for free, that saves a lot of points. Don't think you have to deep strike every time- remember that they can just walk on from the table edge if you wish. If they're needed close to your table edge, just walk them! I know this sounds like common sense, but it's easily forgotten when you want to use their abilities (similar to how people will sacrifice movement to get a few shots off).
Charging with your Warp Spiders is another good trick. To keep them safe from shooting for a turn, you can launch them into an assault, their high armor hopefully keeping them alive. If you upgraded your Exarch you may even take a few down. I have found that against all but dedicated assault units, Warp Spiders can hold their own, finally using Hit and Run to get to safety.
As for the Exarch, I always give him the double spinner, powerblades, and Hit and Run. Any extra shots I can give are worth it, and seeing as I often use their melee abilities to harass and lock down units, they might as well take some down with them. For unit size, I try to go with the max. A small unit can be effective, but having full size makes their advantages even stronger. With a large unit, you can take casualties and still be able to make a difference on the battlefield, being effective through the whole game.
I have never experimented with putting an Autarch in a unit. It would be interesting to give him a fusion gun and power weapon, so the unit would have greater vehicle popping abilities, and an intense close combat presence. This is a unit that can pop transports easily and take out the squishies inside.
I'd like to hear how everyone else uses Warp Spiders (or if you don't use them at all, why).