I have been using only Vista for a while now as my primary OS. I have it on my work laptop and desktop. Also, I do all my coding on Vista. So far, I have to say, I really like it. It has not crashed, the side bar is really cool (especially the stock ticker gadget). The animations and the UI are cool.
The LUA(limited user account) mode can be a little annoying at times with several dialogs to wade through, but I have grown to like it as it tells me exactly what and who is attempting to install anything on my machine and whether I want to go ahead with it or not. It gives me more control on whats on my machine.
Also, the vista perfomance index can help you back your claim for a machine upgrade at work:)
I have not really looked under the hood to know how Vista is an improvement over XP, but so far I just feel a lot better on Vista than on XP.... maybe its just a perception, maybe there is a lot more to Vista than most folks think....