The interface is a three pane window. On the left are your library and playlists (Smart Playlists included). In the middle are your songs. In the right column is the old and new metadata. Select a song (or songs), click "Check All for Tags," and wait. Songs with found metadata show a badge in the top-right: a circle with three dots. Songs that can't be changed or that aren't found show a badge of a circle with an exclamation point.
While you could blindly trust Tagalicious to update all the metadata, I suggest checking through the results. I found a few examples where it suggested changes that I didn't want. You can easily select which information you want to update and which you want to keep as-is. (You can even mix-and-match some new data with some old data, however you can't type in changes manually.) When the information is as you'd like it to be, click "Send to iTunes," and the badge is changed to a checkmark.
I've been using Tagalicious for several hours, and while it does have some rough spots (a few crashes here and there, usually when trying to check a large number of files at once), it has very much impressed me. I'd say the artwork and metadata has been about 90-95% accurate. Tagalicious also supports lyrics, which is surprising since the record companies have been aggressive in going after websites that compile lyric data. That said, the feature is pretty hit-or-miss. Some songs show complete lyrics, some are truncated, and some are just not there at all.
Occasionally, I'll see a song for which new metadata/cover art isn't found, even when other songs from the same album are. Currently, the label for "Music" under Library includes podcasts and audibooks. I just wanted music files, so I made a smart playlist and set Tagalicious loose on that instead. I suspect that these and other minor nits will improve as the application is developed further.
The demo version will let you check 50 songs before buying. I threw 50 of my most challenging files at it (ones that did not already have cover art or lacked some metadata), and it found results for 48 out of 50. Of those 48, maybe two to three contained some errors. That was enough to get me to buy it. I have been using TidySongs, but it checks files one by one (rather than batch-lookup), which gets very tedious very fast. It keeps telling me that iTunes isn't responding. Supposedly, there is a new Windows version available, and a Mac version has been coming "real soon now" for a few weeks, but I got tired of waiting. Tagalicious works much, much better than TidySongs, and it's half the price. Plus, Tagalicious is a native Mac app instead of an Adobe Air app like TidySongs.
The cost for a single computer is US$19.95 or $29.90 for a five-computer "family pack." (You can upgrade the single computer to a family license by simply paying the difference later. I keep my music on one computer, so that was sufficient for me.) A single-computer license for Tagalicious is $50 cheaper than SongGenie ($30) and CoverScout ($40), and the family pack is $70 cheaper than SongGenie and CoverScout. Buying any sort of metadata/artwork cleanup software is done to save you time compared to doing it all manually. SongGenie, CoverScout, and Tagalicious all have demos available for you to try before you buy. So far, I'm feeling pretty happy about the results I'm getting for a much better price.
Which reminds me, Tagalicious also offers a 10% discount via their Facebook page.
Despite some rough "version 1" spots that I hope will be smoothed out in time, Tagalicious seems like a great utility at a great price as compared to the competition.