OK, the first time, I had almost all of my songs rated, then I found out the hard way that it doesn't store ratings in the tag, fine, my mistake.

Just now, the second time, I needed to add some songs that I have downloaded/ripped over the last week or so, but I forgot which ones they were. The simplest method is just to delete all the songs and then just import my MP3 folder. But this time, I wanted to keep my ratings, so I exported the library. I then added all my songs back in, imported the library - no ratings!

In all, I have spent about 2 hours rating songs which have gone down the drain, all because they didn't take the route of having the ratings in the tag, thankyou Apple, thanks very much.