I've recently upgraded from SONAR 8.5 to Platinum. One of the nice things was still getting upgrade pricing after skipping several updates.
I'm a hobbiest, and decided to take another whack at recording after some years away. I upgraded my PC after 8 years, and updated my SONAR rig.
Some observations:
Pro Channel is awesome, and with the expansions come close to a "do all" plug in. The Platinum plug in package is close to everything that might be needed to do quality recordings. I am looking forward to the updated bus eq and multi band compressor. My updated Waves plugs compliment the Cake plugs nicely. Current ROMplers (Dimension and Sampletank3) may be better than my (hardware) JV synth. Current plug in reverbs may be better than my (hardware) TC reverb. Melodyne is almost worth the price of the upgrade alone. Hardware integration has not suffered - my mackie control, Tranzport, xL one, MOTU interface, and XP-30 all still work with the new version.
I AM wrestling with skylight, but think I will get there.