The warlock I'm playing at Encounters is one of the most fun characters I've ever played. Of course, part of that is because I first made her as a wild mage in Hackmaster 4e, then briefly as a warlock in D&D 4e until the wild mage released, and now she's my highest level active character, so she lives in my head with the rest of the cast from my book. It also helps that's a bit nuts. 8o) I honestly can't think of a character that has been more fun to roleplay.
But the subject at hand is mechanics. There, too, she is proving to be a blast – sometimes literally. We are currently level 3, so I have two level 2 spell slots. My first is pretty much always Hex, which may well be the only concentration spell she ever learns, and so far Shatter has been my most-often cast spell in the other slot. It has proven to be especially effective in the tight underground spaces in which we keep finding ourselves.
Given Shyui's explosive personality, the obvious incantations are Agonizing and Repelling Blast, so her Eldritch Blast is 1d10+3 and pushes 10', leading to all sorts of fun. I accidentally killed one of the bandits by launching him into a tree. This week we were in the moving stones room, and Shyui gleefully and repeatedly made the idiot regret trying to hide between the eponymous stones. 8o)
I still had Hex up for that battle since it had been less than half an hour since I activated it, so when I could get line of fire on him I got to deal that extra damage, and when I couldn't I got to hit him with a bloody big rock. 8o)
Shatter has twice now let me lay waste to a small group of minions and a chunk of the dungeon. The first was when a group of skeletons was stopping us from descending some stairs. From my position I couldn't see them very clearly, so I Shattered the ceiling above them instead, killing most of them while keeping the flying degree out of reach of my allies. It was glorious.
Honestly, the only caster I've ever had more fun with is my shielding swordmage.