Rogue Audio Cronus Magnum - Worth spending $700+ for Mk.III version?

Really all generations on the Cronus are nice, but as you move up, you generally end up with a lower noise floor/background and improved phono stages and headphone amps.  If you don't plan on using either of those (phonostage and headphone amp), then the Gen II tends to be the best buy on the used side in my opinion.
 

I would somewhat disagree with the above post....I love the CMIII with KT88s, and actually would sometimes run 88s in CMIIIs even when they were shipping with 120s.  I have several customers that run both, and both have their positives.  

MC capability was the main feature, but the stage was supposed to have been improved in general as well.  With that said, I've never really done a back to back with a CMII versus CMIII only using the phonostage to see if you can tell a noticeable difference.
If you don't mind spending the few extra bucks, then I would say certainly the CMIII is best of the generations.