I have the U-bass. I used to play a short-scale Supra bass, many years ago (a 25" scale). The U-bass is fun and has a nice, woody sound when plugged in (almost no sound unplugged but fair enough for quiet practice without accompaniment).
Advantages: size, short neck, lightweight, cool looking. Standard tuning.
Disadvantages: strings take a while to settle and stay tuned (like ukes, but better than the Ashbory which never really settles and continues to stretch). Strings are fat and rubbery, so they tend to roll under your fingers - you need to work on a different technique when playing to avoid this (not difficult but needs a bit of concentration) - escpecially when sliding. There are no active controls, and I find the output somewhat unbalanced in favour of the G string. Needs some moderation via the amp to sort that out. The short spaces at the higher frets make it more difficult to play above the 12th fret.
It's been fun re-learning bass. However, I'm looking at changing to a different style - either a Kona Walking bass or an electric upright like the Dean Pace. Might sell the U-bass to help finance one.