I konw I'm still learning, a good few player take double Slaanesh DP with the lash, are they really that good......

I have played both Eldar snf Nids that both have items that make you throw 3 dice for psyker test and its not much points against mech armies.............

comments welcome

Things that make the lash suck;
Good anti psycher gear
All mech, small disposable squads inside 10 tanks.And now being able to score sat safely in the tanks.
Everyone at the tourney taking dual lash and spending the whole game lashing forwards and backwards.
Killing the DP turn one with mass str6 shots (eldar speciality although chimeras etc can do it too)
Bad dice rolls, only rolling 3 inchs from 2 lashs will give the game away.

Things that make the lash scary,
Not being able to take any anti psycher gear (tau,chaos,necrons,orks)
Not being able to take an undercosted and superpowerfull all mech build.
You also get to be int6 (for free basically)

The lash can pick up my shrike wing/nob bikers and dump it into difficult terrain (so I have to dangerous terrain test as I'm put there, take a pinning test, take a blast to the face and then take a difficult terrain test if any are alive to leave during my turn.)Ljundhammer did that twice to a squad it went from 10 man to 3.

The lash may grab 30 loose coherancy boys/necrons and podge them into a perfect circle further away from you (and out of range of that kustom force field, ressie orb)and have pie plates land (getting 20 or so hits each)

If going second and it lasts that long the lash may pull opponents off their objectives, stopping them holding/winning the game.
My speciality, you conveyor belt, so that you optimise LOS. You have hidden shooty units apportioned to get cover/not shot by the enemy, your prince leaps forwards..drags unit into range of squads A-C they shoot the snot, prince B leaps forwards lashes 'down the conveyor belt' to units D-F to shoot a bit and assualt the lone guy, this works incredibly well against elite heavy/low model count/death wing lists.The remaining army can only see one demon prince (possibly) at the start of the conveyor belt, because at no point did you need to see where they were..ony where you wanted them.

(few more, moving unit to a place they can't regroup, or are trapped-venus fly trap, pull them towards beserkers/defiler for turn 1 charges, I quite liked my dual lash with defiler, I called it the orbit..it just sucked once ironclads hit the scene)
Thats a few of the more super reasons its awesome..alot of gamers are ready for it, take counter measures, but by having to counter it they have weakened their armies..I am still umming and arring about a whirlwind (for ork nid swarms) or a librarian with hood (for lash/jetlocks and demon armies).

I tried the 2xlash thing a couple of times to see what all the fuss was about (borrowed the club CSM army). First game my opponent took Eldar and the warlock squad of doom, stuffing up my psychic rolls and making a nigh on indestructable unit. I lost.

Second game was against daemon hunters and I won easily. I think as you and KBM have eluded to, some armies have natural defense against it, others really do not.