what will I gain if I go from S6 to S8.5

Sonar 6 is working alright. Is it really worth moving up to Sonar 8.5

Totally agree with Blades on this one. There's no comparisson between 6 and 8.5 really. Just about all the ways you do stuff in 6, you can do in 8.5 faster and easier. It's loaded with more options. One of the cool things you gain is the ability to hi-lite tracks, click on an output and you now see "selected outputs" which means in one click of the button, all the outs you have selected will change to where you need them to go. In 6, if you had lets say a big drum kit that you wanted bussed, you'd need to select each individual output and send it to that bus. Now in 8.5 (this feature was added in 7) you select all your tracks and just send them in one shot where you need them to go.

Arm on the fly is also a cool option. You no longer have to stop Sonar from playing audio to arm a track. The new plugins are fantastic...especially Vocal Strip, Percussion Strip and a few other goodies. As Blades mentioned, Dimension Pro is really killer. It will definitely be one of your main softsynths as it is loaded with quality sounds that just about anyone from any genre of music will find useful. Session Drummer 3 is also a nice addition. To be honest, this new Sonar is so powerful (and very stable for me I might add) you can literally create music without knowing how to play an instrument and have it sounded totally killer. That's how loaded with options it is. So check out the new stuff in 8.5 and compare to what you have now....I think you'll be quite impressed. Good luck. :)