I bought a copy of SX Studio Earlier in the Year to Run along my Pro Tools Setup.
In comparision to SX3, I don't think there is too much of a difference. Steinberg have mainly invested on improving the way project and presets can be organised. Its alot easier to recall and store settings. On the Full version you are able to use features such as the Control Room set up which apperantly allows you to set up headphone and alternative mixes much easier (better hardware integration)
Cubase has also updated its plugins, essentially giving the old plugins a nice new GUI and its added few new toys. I can't really say they are anything to write home about though. The way Cubase looks is a bit nicer to. More sleeker colours. One thing. Cubase 4 doesn't support Direct X anymore. Only VST, so say goodbye to any plugins you might have..
As for buggyness, well I get the odd crash. But I havent really witnessed anything that I didnt get on SX3, just make sure you keep updated.
All in all, I think that if your not fussed about getting a few more synths and a sleeker look, its probably worth hanging onto your copy of SX3. I was really hoping for Cubase to be on par with Logic, as in feature and giveaways.. However I was slightly dissapointed. I think its getting there though. Also I have a sneaky feeling that Cubase 5 is only just round the corner. They seem to release a new on every 2 years...