There are several posts about build 518, and i'd like to add some observations.
I'm running XP/SP3, and did some heavy tag editing with 518. I've also synched my iPod Classic 160GB (latest firmare) several times. Auto-Import is disabled, but i've imported a lot of albums with 518. So far, i didn't see any library corruption (knock on wood!), and the iPod database seems to be in order, too.
What i did notice, however, is that MC adds a "replaygain_track_gain 6.000000" tag to _every_ mp3 tag it "touches" (writes, updates). This value is not visible in the "Replay Gain" field inside MC's tagging window, but it can be seen, for example, by playing the file with Winamp, or by viewing it with a hex editor. The tag is written no matter if the file has been analyzed, or not. I hope the next build will remove these "bad" tags automatically, otherwise i'll have to wade through a _lot_ of mp3's to get rid of the incorrect tag.
Another thing i've noticed is that cover flow on the iPod is behaving strange. I'm not 100% sure, but i think "multiple artists"-albums were grouped together in cover flow (i don't use cover flow too often, mainly to "show off" (; ). Now, i have multiple entries for "multiple artists"-albums, one for each artist.