The Review -
Ah the standard Eldar Aspect with the slightly better armor than normal and improved movement fun. Warp Spiders have the usual good offensive stats with poor defensive stats though this is shored up with a 3+ save (still T3 though). They come with Warp Generators, (aka Jump Packs) for good mobility which also allows them to make a 2nd move during the assault phase. This is a 2D6" based jump however so not as reliable as a set 6" (thought at least it's 2D6 so you've got a nice bell curve). They also carry
Unfortunately, it all goes downhill here. The guns are 12" - yes 12" and AP -. This means they need to get close to be effective and that good strength isn't nearly as useful against vehicles as you'd expect, particularly with the mobility of the unit enabling side/rear shots. This combines poorly with their unreliable 2nd jump as well - you could jump in and out consistently for 11-12" but it's more likely you'll roll poorly at one point and get caught in combat and despite their 3+ save, Warp Spiders don't like combat.
Their Exarch can help here though thanks to Withdraw (aka Hit & Run) - they can escape combat if they get into it - he won't make them any better despite Surprise Assault (+2 on charge) or power blades (power weapons, +1A). He is a good buy regardless as he can bring an extra deathspinner or the spinneret rifle for improved damage against tanks due to AP1 - best bet is the double deathspinnner though.
Potential Uses -
Not much that Spiders are really good at. They aren't cheap and don't really shred infantry for their cost or tanks and suffer from a really short-range with an unreliable 2nd jump.
Their most basic use is as a flanking force where they can get to side/rear armor of vehicles (AP - hurts this though) or harass backfield units. Using Hit & Run they can engage such units and not take much damage to avoid being smacked around by shooting and then disengage next turn. They put reliable torrent wounds on infantry with BS4/S6 and that's about it.
They can be kinda nice with an Autarch attached and the Exarch running the AP1 deathspinner - this gives them some legitimate armor breaking power and if they get stuck in combat they have some staying power with a handful of power weapon attacks backed up by Withdraw (Hit & Run). It's expensive as all get-out though and still pretty inefficient and it's basically just an extension of what was outlined above.
Potential Changes -
Make their second jump a set distance - i.e. 6". Points reduction. Improve their weapons so they can be used from afar (i.e. standard Rapid Fire 24" with Relentless or Assault 2 24"). Give them a damn Phoenix Lord, particularly if there is some sort of scoring bonus associated with all the Phoenix Lords and their respective Aspects. Remove AP- from weapons (AP6 is fine, as long as they aren't screwed versus vehicles).
Warp Spiders have little use in today's environment. What they provide, S6 firepower, is basically provided by the entire Eldar force at greater ranges. The only thing Warp Spiders do is bring more of such firepower per point yet due to AP-, their short-range and weakness in combat, they aren't an effective choice. They can be used to race up a flank/deep-strike and engage backfield units with Hit & Run but that's really the extent of their use. If this were expanded in some way so they are more useful on their way in and more reliable in what they do (i.e. 2nd jump), they could be a much better harassing tool.