Ubuntu Linux boots in seconds. Just works.
The install was easy as could be. The OS boots up very quickly, and shuts down in seconds. The Open Office Suite apps are excellent. The loaded music program is basic, but quick and simple as was the DVD/video watching software. There's plenty of legally free useful software for the OS too.
The Firefox browser bundled with both versions is as good if not better than IE.
Great.
What's not so great is configuring things like modems and graphics cards to work properly, nor is it easy to troubleshoot problems. In fact, to anyone raised on Windows it will likely be a confusing, complicated nightmare.
Like running command prompts? You'd better, and you'd better be willing to spend gobs of time learning new ones, along with an OS structure significantly different from Windows. Also, the default password nags are horrific out of the box -- worse than Vista UAC. I worked for three hours trying to install a .tar install package for a wireless broadband modem -- it was a convoluted nightmare and I finally gave up -- after spending hours on the Ubuntu forums.
My final assessment?-- there is a serious learning curve to Linux, even the "user friendly" ones with a GUI. I'm an advanced hobbyist who loves a challenge, but within a few days my foray into Linux became an exercise in frustration. Most ordinary computer users aren't going to bother with the hassle of getting Ubuntu tweaked to their needs.
I'll wait a few years before futzing with it again. Maybe by then they'll have made it easier and more intuitive to use and easier to tweak. For now, XP's my choice. I just run it devoid of crapware and eye candy and it runs quickly and reliably.