I've always loved the sound of a good fretless bass. They allow an incredible amount of control over the envelope of the sound from the shape of the attack to the sustain that you just don't get on a fretted bass. It begins with the singing tone referred to as "mwah" by bass players but goes well beyond that. All of that control requires... control on the part of the player and there is the issue of intonation. With no frets, moving your finger a fraction of an inch takes you into or out of tune.
I've owned two in the past. One simply was a lousy instrument and nobody could have made it sound like what I hear in my head. The other was a decent bass. Of course the real piece was that I wasn't really a bass player when I owned either one. Over the past few years, as I've been playing bass in the church band, I'm becoming a real bass player rather than just a guitar player, playing at bass. I'm hoping to have some more opportunities playing bass as a backup player for local singer/songwriters.
A second piece has come into play. Bass guitar builders are much more adventurous than guitar builders and bass players likewise. This allows the builders to push the envelope a lot further than guitar builders from interesting designs to multi-string extended basses (some of which have as many as 21 strings - here's a link to a video of a Bee Bass 9 string from NAMM if you're interested).
|Bee Bass GrooveBee|
I'm in love with it even as it challenges me to be way more careful of my technique and aware of my intonation which was never even an issue with fretted instruments. I'm working at both. The bass inspires me to work at both so that makes it easier. Here's a video of my bass made by the builder... you can hear the mwah. What you don't hear is how aggressive the low end can be.