The first thing I noticed about the TuneUp plugin for iTunes is that it isn't. Isn't a plugin for iTunes, that is -- it's a separate application that 'side-hugs' the iTunes window, and tries to track with it as you resize, hide the Genius sidebar, etc. Most of the time it's not much of an issue, but you do notice the lag and redraw oddities when moving the iTunes window around, and it's a reminder that the integration between the two apps is less tight than it would be with a true plugin approach. There are other rough edges in the TuneUp 1.0 version, mostly annoying rather than serious; the one that kept me gritting my teeth was a persistent marble of doom that hung around long after the tool was done processing. You could click through it and keep working but it definitely was harshing my buzz.
TuneUp's concert search, video pane and cover art download features are nicely done (I especially liked the YouTube inset for selected songs), but the core of the app is the Cleaning. I dragged a cluster of 80 songs onto the Clean pane and let the tool start cranking away (it took several minutes for all the results to come in, which is normal). TuneUp broke the results into "Exact Match" and "Likely Match" sections, presumably based on the strength of the Gracenote fingerprint, and I was pretty impressed; it showed corresponding albums and the matching tracks from a bushelful of well- and lesser-known singing groups from colleges around the country. Once the matches came in, I clicked Save for individual tracks (or Save All for the batch) and the iTunes metadata began to update on the fly. Cover art was also available for most of the tracks; clicking the thumbnail gave me the choice of which images to associate with the tracks.
Out of the 80-odd tracks I chose, more than 10 couldn't be identified at all; your mileage may vary, but a cursory check showed that most of these were live or other bootleg recordings that were unlikely to be in Gracenote's files anyway. For the most part, the identified tracks were right on the money with artist information. The one clear error was in associating one version of "Stay (Wasting Time)" with the authentic Dave Matthews Band rather than the a capella group that had recorded it.
More problematic (at least for my iTunes usage profile) was that TuneUp was quite thorough in replacing the track metadata with its discovered, correct information. This thoroughness extended to the Genre field, which meant that many of these tracks -- unclassifiable by Gracenote's standards -- ended up as "Data & Other" songs, while others got obscure genres I'd never thought existed. Since I was using the genre of 'A Cappella' as a tag for all these songs, I had to go back into iTunes, sort my tracks by modification date, and change them all back to the correct setting. You might not have this particular issue, but for me it wasn't that pleasant to have to 'undo' the work of TuneUp. A preference setting to allow selective blocking of metadata changes from key fields would probably clear this up.
Overall, if you have a messy iTunes library and you crave some organizational assistance, you probably should try out TuneUp's free version on a few tracks and see how it works for you. I'm not necessarily sold on the full version myself yet, but some work on the cosmetic and performance issues combined with more options on metadata replacement might turn me around.