I tested out 5.0 on my PC (haven't touched my operational iTunes library yet). I cleaned out the old library and dropped a folder of 3086 songs on it.
It failed to correctly attach the metadata (artist, album) for about 8 of them. In iTunes 4.x the same files show correct metadata.
I've not seen this problem on prior versions of iTunes/PC. I'll hold off on updating my main iTunes setup (Mac) for a while.