Not sure how many play about w music apps, but Native Instrument's iMASCHINE
is on sale for another 1.5 days for $1.99
down from $4.99. Not too bad.
Even though it's ported for the iPhone and doesn't have true iPad support, i've been told it's actually alright on the iPad. But at this price, i just grabbed it and will wait for the iPad supported one to drop.