Network Computing has a nice comparison between Vista and Ubuntu. I’ve yet to even see Vista, really, but I can say I was disappointed that they didn’t include DVD playback with the multimedia testing. Due to the proprietary encryption with any DVD playback, free and legit Linux distros tend to not be able to do this out of the box. I was happy to see mention of Ubuntu’s occassional (and very frustrating) hardware issues (namely wireless or sound issues from what I’ve heard) which can send people back to Windows quickly.
I think Ubuntu is a nice alternative for light users who don’t install their own things and only need major things like email, web browsing, maybe some IM, music, picture viewing, and office productivity. Basically you don’t need much more beyond what is installed by default. If you need more, you might be in for some learning curve issues.