I DM'ed a party with a 7th level warlock. She had the ability to spiderclimb and turn invisible at will. Thus every round of every combat she did the essentially the same thing; be up on the ceiling and blast. Occasionly turn invisible and move if something climbed up after her or an enemy spellcaster took an interest.
I found that without specifically crafting opponents to counter her logical tactic, she sat up there slowly dealing death with relative impunity. The warforged fighter in the party may have dealt more total damage, but her character more easily toyed with typical dungeon challenges. Thus I wouldnt necessarily say warlocks are overpowered, but they can be a handful for a DM or a store bought adventure module.
In the end however, the player herself was the one who did in warlocks, not I. She found it rather boring to do the same thing every turn of every combat. "I'm up on the wall and I blast." Of course with different selections of invocations you'll get different mileage, but with such a small number of invocations warlocks are something of one-trick ponies. That could be fine if your phoning in your performance or your not so interested in the board-gamey aspects of combat. But our group found that the mantra "actions are the fundamental currency in the D&D game", is true not only for character power, but player enjoyment as well.
Personally if I were to exist in a D&D setting, warlock would be one of my first choices of class, which should tell you something is odd...
