I switched from Vegas for several reasons when it was CS5.5, but CS6 has confirmed that it was a good move as it is more stable and now supports RC3. Here were the original reasons:
Red Rocket support
Stability (although this proved not to be true as CS5.5. was as unstable as Vegas 11 x64 if not more. This has improved a lot with CS6.
Better support for third party plugins like Boris and Red Giant.
Unfortunately there were contras, notably
Premiere's piss poor audio capabilities and appalling VST support (can't specify the VST folder)
Premiere's cumbersome editing interface, which is nowhere near as intuitive as Vegas
Plugin interface support and generally kludgy plugin implementation
Constant hassle with Adobe Media Encoder and its frequent crashes
Crap autosave implementation
That stupid thing that happens when Premiere decides your hardware isn't up to scratch and so it quits the app without letting you save the work
Finally, after over a year and an upgrade to CS6, I can say that I'm glad I made the move. Render times have sped up enormously, AME is slightly more stable, finished renders look beautiful, and overall stability is improving. The VST management is still rubbish, and the insistence on using Premiere's interface for all plugins is still a crap idea. New Blue Plugin automation is next to impossible. And now I have autosave set to every 3 minutes and up to 250 versions.