I'm still using Vegas 3. I never saw much of a reason to upgrade. The bus to bus routing is something I've always wanted, but not worth the price of Vegas 5 (as it would be the only reason I would "upgrade").
Everytime I've read a debates about the mixing algorithms, one guy will say that Vegas' algorithm sucks, the next guy will say that Pro Tools isn't that good, and another guy will say to use a summing bus. No one agrees and when that is the case, the differences can't be that different.
Brandon