I ran out of RAM, the iMac crashed hard, and my iTunes Music Library file became corrupted and unusable. I'm not a big stickler for application-specific metadata, but tune ratings are an important part of my iTunes/iPod experience. A quick trip to google yielded this tip page, which is a bit dated but still valid.
What it boils down to is: if you have a valid .xml file but not an iTunes Music Library file, import the .xml file and you'll be fine. If you've got a particularly large .xml file (mine's 16 megs!) it'll take quite a while to re-import.
On the one hand, it's a pain. On the other, Apple could've easily kept the file format totally closed. By choosing XML, they saved me quite a lot of time and effort.