Slaanesh spells are good, especially if you have a Hellcannon and a BSB with Doom Banner, and mostly Khorne units, imo. Reason being is that a lot of the spells are meant to deny enemy movement in some fashion, or outright kill them.
However, the Mark's benefit is totally useless. Tzeentch is easily the most beneficial Mark for our Sorc Lords, followed by Nurgle. Khorne is terrible for casters and Slaanesh is essentially useless. That being said, here's a run down of how I use Slaanesh spells:
Lash of Slaaensh = enemy unit potentially can't march next turn = another turn of Hellcannon shooting.
Hellshriek = panic. -1 LD from Doom Banner = easier panic = run away = another turn of Hellcannon shooting and/or making other units flee, potentially off the board. If they don't run off the board you can rear charge and get really good CR.
Hysterical Frenzy = free frenzy. Chaos Warriors have T4 so taking S3 hits means you're probably not losing any Warriors.
Titillating Delusions = amazing redirection. Throw your opponent's units off course, screw up his tactics, expose their flanks, force them to charge your Khorne Chosen for a bloody good time, or even use it to make them block their own units from charging yours. Such a great spell!
Aura of Acquiescence = your stuff is harder to hit or won't be charged at all.
Ecstatic Seizures = considering the avg T of units is 3, this spell rocks! I like it more than Infernal Gateway, especially against annoying bastards like Phoenix Guard! TAKE YOUR WARD SAVE AND SHOVE IT!
Lastly, 2 of the spells are RiP, so if your opponent either has to waste PD dispelling it and wasting his own magic phase, or he ignores them and faces the consequences!