I played with Rebirth on and off pretty much since it's been around and my opinion is that its sound is an homage to the real 303, at best. The spirit is certainly there but it's hard to make Rebirth sound exactly like a real 303 or hardware clones. I haven't played with any of the newer (as in, made in the last 15 years or so) 303 software implementations but I would suspect most of them are more faithful recreations.