I upgraded from Sonar 4 to Sonar 5 last year, and I don’t think I really got much out of it, feature-wise. I got more out of upgrading from Studio Edition to Producer Edition.
In fact, Sonar 5 turned out to be pretty buggy. I’m not sure I want to upgrade to Sonar 6 if it’s unstable, even if it’s tricked out with new features. This thread from the Cakewalk forums, however, does mention some things that sound tempting, namely audio quantize.
If I do decide to upgrade, I’ll do what I’ve done the last few times — wait for a sale. Christmas is coming soon.