The TweakyBeat update I've been waiting for is finally here. Version 1.7 adds Midi Sync, AudioBus, and Inter-App Audio so TweakyBeat is going to be just so much more useful as I'll be able to get it running alongside SeekBeats and Elastic Drums too.
But even on its own TweakyBeat is an awesome app for making synth drum sounds, and I'm not sure that you're going to find a better use of $0.99 today. So if you haven't got this app already I suggest you grab it. For $0.99 it's a total bargain, even more so now.