Now this might sound crazy..but i just used WS as starting unit against space marines..and won 5 games in a row..
1v1 ladder 1200 point mind you,no noobs..no experts either,but ok players.
several points that make them useful - start game scouts killing.They always run from guardians,well good luck outruning those suddenly warped in WS..
great defense against scout rush
Ok hero damage
resistant to bolters.even heavy ones.Granted the deathspinners take some time working on marines,but WS squad -can- take standard tactical squad head on and win if reinforced in full.Didnt try on heavy bolters but they'd prolly lose that.And deathspinners do work them down faster then say guardians..
+much better with exarch
sniper killing!!! warping + bolter resistance means you can warp behind his force and take all the snipers down in secs - especially if banshees are already hitting the front - just a little micro since scouts are small units and sometimes take some effort to get selected as target.but not that bad..if he tries to snipe you out of position you just warp his scouts everytime he tries..
and maybe mostly importantly,morale damage to enemy.Those SM guys dont know how to counter WS because they rarely see it.I've just had 3 of those guys build assault marines squad,apparently just to attack WS.That didnt work because i warped and in confusion the assault squad just settled for closest banshee squad - and got wiped out after some close combat.
There's lot more like grenades against vehicles and suprise raids,this squad just adds the tactical flexibility eldar's lack a bit.Think of it,correctly placed tactical squads will mow down the banshee assault,the dark reapers die all too often to assault squad jump or FC or they have to run around and thats bit expensive fun for 400-510 cost unit.The banshees are food for snipers etc.
But with this unit you can screw the situation up.Its not an hard counter,but its still pretty effective unit even against SM.Its also quite easy to micro - no FoF turning on/off or dancing,just warp and gun.
Well this got little too long.Anyway.Iam not saying the WS are some salvation against SM.But,since SM players dont usually know how to play against them,they are great suprise weapon.Give them a few tries and you'll see of course,once they get used to them they'll find counters etc - but for now,heck i love em.