I played a warlock from levels 5-10, and yeah, it's a fairly well-balanced class. The spell-like abilities at will were nice, but they never overshadowed the arcane casters in the group (for a while, there was also a wizard and a sorcerer, so there was plenty of room for comparison). Also, the invocations are a pretty good mix of offense and utility.
Plus, there's all sorts of good roleplaying potential built into the class.
Demiurge out.