There are a lot of apps that compete in the 303/808/909 emulation space. technoBox2 takes the top spot for the best sounds combined with a full techno ensemble. 2 303s and 2 Drum Machines, perfect for any basic acid/techno/house track. It lacks the ability to pan anything, and drum machines have a global LPF. Effects are also, sadly, global. A lack in MIDI Clock Sync limits its use with other apps and external sequencers. There is also no AudioPaste support to get samples in, but you can use iTunes import to add your own drum samples. See technoBox2 in action in the first 5 Lessons of Everyone Can Play Music. If you need MIDI consider bleep!BOX or BassLine together with Molten or MoDrum as alternatives.
It has been over a year since this was updated last; it is increasingly unlikely we will ever see the promised MIDI Clock Sync. This app has not kept pace with the competition.