I've been a Beta Tester for C7 on OSX. It runs well. 

I also use modo which is really a different animal than C7. It's aimed at a modeling and photo realistic rendering market so it has some optimizations for that C7 doesn't. However, C7's shader options are much more flexible and you can do nice cell shading renders (Toon) and NPR renders that you can't with modo. So, if doesn't break the bank, C7 is very nice to have around. I use both - and others too. Whatever gets the job done.

Mark