Tried it for 30 min w iPhone 3G: Looks great, sounds impressive analogue, great and clear (using Sennheiser MM50 headset), stable app. Seems to be the "real" Roland Transistor Bassline 303 enhanced with two nice effects pedals. Quite usable method to move the knobs on a touchscreen. Programming the sequencer like the real thing. Perfect emulation.
Good tutorial on their web site.
http://en.wikipedia.org/wiki/Roland_TB-303
Get it if you like those house music acid sounds. Almost unbelievable: The complete sound studio in your shirt's pocket!
Thanks for the hint. Have to leave for work now and will try to compose some bass lines in the subway ;-)