A Bass Uke will NEVER sound completely like a true Upright Bass. Let's get that out there first. Nor will it sound like an electric fretted or fretless bass. Each of these are their own instrument, and knowing the nuances and techniques of each (because they are different, despite being strung up the same) is going to benefit you more in emulating each of them than the instrument itself.
However, the Kala U-Bass has a very warm, robust sound that gets in the area that many of us say is "close enough," and frankly the audience rarely cares. I've only had mine for a week, and tonally it is pretty close to my upright bass. I brought the U-Bass to my folk group rehearsal last night. The sound blends very well with the acoustic guitars.
Do they have an advantage over other basses when playing with ukes? I don't think so. The player has more influence (style, note choices) over what instrument they're playing. Dirk from Southcoast Ukes is working on a "piccolo" acoustic fretless bass, citing that the tonal range (one octave up from a normal bass) will sit better with ukes. I'm very curious on that, because it makes sense; there is a LARGE tonal space between a bass and a ukulele. However, that also allows both musicians a lot of room, if they know how to use it properly.
Good luck if you go the fretless route. Many people have a hard time with regular fretless scales that trying to cram your fingers on a 22" scale to be properly in tune isn't worth it. If you check the
Gollihur Music site, Bob even recommends AGAINST buying the fretless one. I played an Ashbory years ago, and quite honestly it was a pain to play in tune. And my normal instruments are upright bass and fretless electric.