I too am interested in the idea of the warlock, but I also worry about the one-trick-pony situation, and how the DM would deal with this. It seems to me that a good DM will let the PC have his fun, and get to use his abilities fairly often. If others are right, then this might mean that the warlock PC overshadows the others a little. But sometimes the DM will need to plan to counter the warlock's abilities, maybe to make an encounter work. Without much to fall back on, the warlock may feel useless in these encounters. I guess he could still blast away, same as a fighter who just keeps hacking.
As an example, I'm currently playing a sorcerer in City of the Spider Queen. The make the adventure work, certain types of magic had to be limited. For example, teleport does not work well, if at all, in many parts of the underdark. Other forms of magic are hindered. But my sorcerer has enough alterntatives that he can fall back on spells that do work, and come up with creative solutions. I wonder if a warlock would have enough alternatives available to him to continue to be a fun PC in this sort of situation.