This is a common perception shared by many who first see the "at-will" abilities. That asid, the Warlock is actually a very nicely balanced class, falling (IMHO) somewhere approximately around the Rogue, depending on the builds.

It's at-wills aren't nearly as potent as you would expect, although at around level 5-7 the Warlock really seems to shine. Don't worry to much though...it's a good class, but not overpowered by any means, especially once other classes break out the big guns.