The warlock does not appear to be a powerhouse class as far I've read--although having said that, the very fact that his abilities have unlimited uses means that a player just has that one broken power that he can exploit ad infinitum.
Even putting the issue of raw power aside, I do think it's easy to see how a character that can only do a few cool things, but can employ them an unlimited number of times lends itself to a lot of tedious combats. If a warlock picks the "chilling tentacles" invocation, for instance, are you content to see every battlefield become a mass of 20-foot spreads full of writhing tentacles? Again, even a sorcerer who only has a few 4th-level spells, and one of them being Evard's black tentacles, isn't likely to have that effect on every combat.
I think the class overall is pretty cool, but some of the invocations either shouldn't have been added, or should have been bumped up in their grade, or should have a more significant limitation on their usage.