I've been fiddling with them on and off for the last 15 years. First thing is be prepared to dedicate yourself to it exclusively for a couple of years to get a handle on it.
It's a completely different animal from a fretless bass - the shorter scale length makes intonation more precise and most chords are very difficult without altered tunings. Sustain on wound strings is very short (which is why the Vigier has a metal fingerboard and other manufacturers use glass or fit them with sustainers). Usually heavier strings help sustain too, and no problems adapting to them as your vibrato and bending technique are back and forth rather than up and down.
As inspiration, Download this to listen to these tracks from Bumblefoot:
http://ratcliffe.co.za/sundry/Bumblefoot_Fire.mp3
http://ratcliffe.co.za/sundry/Bumblefoot_Raygun.mp3
(BTW - Yes, I have his permission to post tracks). Most of his 2001 album
9-11 is fretless and covers quite a few styles of music.
And if you're really brave, I have a BBF lesson with the song
Mafalda:
http://ratcliffe.co.za/sundry/Mafalda.rar
That includes tabs, audio lesson, complete track and backing track. Be warned though, it is a complete cow to get right on a fretted, let alone a fretless.
