It seems pretty good - I messed around with it for a while and was able to make some loops that actually sounded pretty good, but the delay between bars that other people are reporting takes away from the experience quite a bit.
The UI is a little non-intuitive, as there are no instructions, but it wasn't too hard to figure out what everything did. I'd recommend adding a preview function of some sort, so that you can hear the sample you're using before you actually play the loop.
With a little polish and a tad more variety, this could be pretty good - I'd like to see you make a few of these for different styles and genres of music. Only giving 4.5/5 because of the inexplicable lag issue.