I picked up a fretless Curbow 4 string in a guitar shop recently, and was pleasantly surprised at how good it felt and played.
Initially, I thought it felt a bit light and plasticky, but after a while I realised that it was actually very well balanced and comfortable to play. Soundwise, it was capable of producing quite a wide array of tones, but I don't profess to having got the absolute best out of it.
If I'd had the spare funds, I'd have bought it without question as a sweet little, good-value bass.