I love it as well. Only problems with it are that sometimes it hangs on startup (press home buttons and lock button until apple logo appears solves that) or else sometimes there is no sound, and a restart solves that as well. I'm sure it is related to what other apps I run before it.
I downloaded a bunch of instruments from the moocow forum and upped it here
http://www.youshare.com/view.php?file=Instruments.zip
I love the rhythm guitar and music box, but the stock 12-bar-blues is just entirely too much fun. If only there was a latch for the beat button... pinky finger is getting tired!