Significant render speed improvements between v7 and v8 series on the Mac. That is actually worth the price of admission in my opinion. Most of the changes between the v7 and v8+ are 'under the hood' meaning it's operational improvements but not a bunch of feature additions. 

There are some bugs, more on the fringes of the program than in the core area. The bugs don't seem to effect everybody equally. I personally have not been working with the Genesis characters in 8.5 so I don't have experience in that area, although I'm aware of Mac users that don't seem to have much issues at all.