I just had a chance to play with it yesterday. I was pretty impressed with Vegas considering the lighting I had wasn't very even; Ultra did handle the task somewhat better - however, the $800 price tag is a bit of an "ouch" and so I've decided to first invest $200 in some better lighting.
Vegas can't do what Ultra can do, based on the ability of Ultra to see and interpolate differences between vectors/marked color areas. Vegas does a great job, but Ultra, like many "specialist" applications, does it better.
So, even with good lighting I should go ahead and get Ultra? Really I was fairly impressed with what Vegas did by using the secondary color corrector, a little chroma blur and then the keyer and getting a decent distance away from my drop - I had assumed that most of the issues I was having were more related to my lighting.
Before you dump your money in Ultra, venture out to fxhome.com and check out a trial version of Chromanator. It has performed miracles for me, and you'll be pleasantly surprised how robust and feature rich Chromanator is.
See one of my video clips done with it on the homepage at
www.daytradingu.com
Vegas can't touch it for keying. SM Ultra has sophisiticated 3d modeled sets that have boom/track pans and so much more ..
Main thing is, ultra is a great keyer.. look at their demo video right now, I liked it so much I made a DVD with it so I could watch it on my home theatre..
I think you'll be happy with it. One thing I love in the Chromanator is the spill supression because I have a limited distance at which I can place the subject so I always used to end up with the green glow, but no more. Good luck