I seriously considered warlock for my new character, but went with a monk. Warlocks seem to me to be archers who never run out of arrows and can't fire more than once per round, essentially. Touch AC for attacks is cool, and you have to be VERY careful choosing your powers and feats. I went with Ability Focus (eldritch blast), Maximize Spell-Like Ability (eldritch blast), and Empower Spell-Like Ability (eldritch blast); with Quicken Spell-Like Ability (eldritch blast) down the road. I took the Eldritch Spear spear invocation for range boost, and Frightening Blast for those big tank monsters. Later, I would have taken Eldritch Cone (that the name?), which turns the blast into a 30-ft. cone with a Reflex save, and Brimstone Blast, which causes extra fire damage. So, then, imagine the level 11 warlock flying in and delivering a maximized empowered brimstone cone eldritch blast for 54 damage flat (or half) to all foes in the area of effect, and forcing all foes to make a second Reflex save or take an additional 2d6 fire damage per round for up to three additional rounds. Not bad...takes a lot of focusing powers to get there, but not bad. Worth it to focus so much? Maybe.
Also, and this is important to note, the warlock has a subtle yet powerful versatility in his own way. Deceive Item allows him to take 10 on all UMD checks, and Imbue Item allows him to use UMD to make magic items requiring spells he doesn't possess.
Staff of fire?
Ring of regeneration? No problem - just take the necessary item creation feat and you're set. Even more subtle - take Scribe Scroll and scribe whatever spells you need, from
any list, as you require them.
All in all, not a bad class. Like a lot of the expansion core classes, the warlock makes a great fifth party member. I went monk simply because when you roll godly stats, it screams "play a monk or a paladin."