I have a fretless Kala U-bass and it does the upright sound really nicely. I owuldn't agree that there are no new skills etc involved in playing one. The rubber strings do take quite a lot of adjustment, as does the short scale.
My only gripes about the U bass are that acoustically, it's redundant. You need an amp to hear yourself. Which leads to the second gripe, it's woefully underpowered in the piezo and even through an amp you have to crank it to hear yourself. Tuning is also a pain.