I've read a fair bit about them, and the consensus is...well, there doesn't seem to be one! I've tried a few Kalas and Gold Tones, and they do sound very cool (plugged in). I'm a bit put off by the quirks of synthetic strings, but certainly not everyone is, and now there are U-Bass-specific round and flat wound steel string offerings, too.
My take: If I were to buy a small bass of some sort, I'd go for one of the guitar-scale acoustic basses. I kind of like those special (spongy) D'Addario strings they use, and they have usable (with reasonable limits) acoustic volume. U-Basses, on the other hand, are really meant to be plugged in; and when you do, there's nothing else that sounds quite like them. So it really comes down to what you want the instrument to do/sound like/feel like.