It will screw up some stuff if you try throwing all of your music at it. My advice, just use it for music that is actually missing info that you need filled and do it in small batches, because you're going to need to check what info it suggests. I ended up deleting a bunch of music I got from friends and re-ripping about half a dozen CDs because the info was so completely wrong and there was no way I could tell what was what. Some albums were split into different albums for each song. Some under a completely different artist and some under a different album.
You know how in iTunes when with some CDs you rip it asks you which album it is and it gives you two or three choices and only one of them is correct? Well TuneUp pulls from the same database. I realize TuneUp and iTunes just has licensed access to the third-party database, but I don't understand how it is just a collection of info from random people going back to the days of Napster and they don't put any effort into cleaning it up to make sure the entries are accurate.
Caveat emptor, it will give you 97% correct info, but just beware that the 3% bad info will piss you off if you don't se it before applying the new tags.