I have a Boss SYB-5 and I find its tracking a bit iffy.
My EH Synth 9 is a nice pedal, but perhaps more of an accompaniment (the Oberheim is very good) than the instrument per-se.
My Markbass SuperSynth (SS) is really good - great tracking, good sustain, configurable via computer. Its downside is it only holds a few presets.
I've not tried the Future Impact, but someone had one at a Bass Bash and it seemed rather good.
The other option is synth pedals; I'm just getting to grips with a Keith McMillen 12-step at the moment.