I used to use Vegas exclusively 8 then 9, then I moved to CS5.
Vegas is very intuitive and easy to use. The UI is easy to learn. It's very good for quick edits, you don't have to spend forever setting up the project like in PP.
Why did I move to Adobe? Well, CS4 was a bit of a dog. Clunky, slow and buggy. But CS5 is a very powerful editing suite. The updates are subtle, but add to make a much better NLE than CS4, and IMO it's just as good as the ever popular final-cut.
I'd say it has much more fine-tuning abilities compared to Vegas, especially in the effects.
If you're a DSLR shooter.... CS5 is pretty much a must-have right now. It's still a total pain to edit in Vegas/Final cut with a Canon. CS5 does it natively and does it well.
Main advantages for Premiere, and reasons why I use it:
- Native HDSLR editing
- Ability to easily mix framerates/resolutions in one sequence.
- Hardware acceleration for special-effects