This may come as a surprise to many people, but I've just now upgraded my main laptop from Windows XP to Windows Vista. Why the wait? Two things: First, I wanted to wait until SP1. Second, I wanted to wait until I had more RAM.
Regarding that second point, my laptop had 2GB RAM, which seems like plenty. However, I was maxing it out pretty easily with XP when running a 1GB Virtual PC (which is how I do all of my software development). So I upgraded to 3GB, which seems to give me just enough breathing room, even with Vista. I would have gone to 4 (which my ThinkPad T60 supports), but discovered that the extra 1GB was pretty much a waste with 32-bit Vista given issues with hardware memory mapping. 64-bit Vista apparently doesn't suffer as bad from this issue, but, alas I do not have a 64-bit processor, yet.
The good news is that on the same machine I was running XP Pro, Vista seems to perform just fine. In fact, it seems noticeably faster than XP was, which is a bit counter to popular opinion. Of course a clean install of any OS always seems to run fast. We'll see how things go.