Nail said:
Sorry... and I don't have the books, so I'm likely to not use the right word... but yes, I think it's the 'essences'.
In particular the bewitching blast and the blast that drains a level (two levels?) for every hit. You've got a weapon that you can use once per round (Eldritch blast) that you practically cannot miss with (2/3 BAB class and touch attack), that you can shoot from either 250 yards away or in a cone or three at once (various E.Blast shapes that you could learn)....
With a high enough Charisma and some gear the saves on those things are far up there. Take feats to increase the DC's of a spell like ability (Eldritch Blast). And you can mass confuse or later mass level drain.
So you can hit a mass of people with a high save DC "spell" that drains levels* if you fail the save, and whether you save or not does a fair amount of damage. That's what was over the top when I saw it played.
(*Or has some other effect. Admittedly this was the "worst" effect I saw. And yes, some things will be immune to such drains, but a lot of things won't be)
Anyhow, I didn't see any problems with the class other than that.
We tried the class out against the rest of the party (the Warlock was introduced by being assigned to kill the party, longish story), he took down the paladin and his cohort (using bewitching blast), a ranger/rogue, and a monk... well, the rogue managed to flee, and the only player who was permanently killed was the paladin, who was true ressurected via a fund the GM had set up specifically for this scenario).
The warlock was also then completely taken down by another monk he didn't see (an NPC monk, a warlock can do NOTHING in a grapple.. he's even worse off than a normal mage).