The system is pretty groovy -- users can define and store up to 8 chords in a palette. Then to play a song, you select the chord from a list and "stum" the virtual strings. You can change chords on the fly and play strings together or individually.
I installed it this morning on my 2nd generation iPod Touch and it works like a charm. It's definitely not the same as having a real uke with you, but I can see it having a lot of value in composing and works great as a time-killing diversion.