I've got reason 4

I was on 3 before that. 3 was important because you got combinators. They made life a squillion times easier, plus you got a mastering suite without decent compressors etc.

I use reason mainly as a VST rewired to cubase and do all the engineering in there, but so the combinator was a massive step up. Mastering suit was nice, made things sound shiny whilst mucking about in reason.

Reason 4 - haven't really got to grips with it yet. There's a new synth that I haven't quite figured out, and a groove mixer that I don't think I'll use, plus a few other bits and bobs.

Really depends how you use reason and for what.