Well, you certainly get some better plug-ins and synths. Perfect space, V-vocal, Pentagon, RXP... To be honest I haven't had much chance to play around with these so whether or not they are worth it to you is a personal decision. The core effects will still be the Sonitus Suite.
Then there are the work-flow enhancements--the track folders, in line MIDI editors, others--numerous posts on these in the past.
There have been improvements to the audio engine. However, I don't think the audio will sound intrinsically much better, unless you really think the "64 bit double precision engine" is going to make a huge difference.
There is no question you can creat great music with Sonar 3. What you need most of all is time, effort, and talent!