I got a Spark amp for Xmas and have been using both a steel string uke (Fanning) and a standard string uke (Godin) with it. I've had it for a couple of weeks.
First of all, it is REALLY fun. Tons of different setups you can use. Direct USB output to use with a DAW. I even took a brief look the capability of getting the chords out of any song on Spotify - certainly looks OK, although I haven't thrown any of my longtime puzzling songs at it.
You can also modify any of the parameters on any of the amp models or pedals. You can do it on a phone, but a tablet makes it easier.
As far as the sounds go, it sounds somewhere between good and great. Some of the setups are obviously for guitars, in the sense they have a lot of distortion, etc., but I have found about 10 that I love already.
One caveat in this positive review is that I have only used it into my DAW or with headphones so far, so I can't comment on how well it does as a practice/playing app.