As others point out, The Tagger is going to have to be good to justify paying anything for it, given that iTunes and now Songbird have the same facility for free.
Certainly filling in tags is much slicker than either of these, and some of the facilities to copy between tags are very useful. (I particularly like the ability to set filenames from tags, which is an easy fix to some horrible messes).
Unfortunately, the user interface needs a lot of improvement; I work with classical music, which tends to expose problems with such packages, and here is no exception; the user interface offers two panes, display on the left and edit on the right, and the sheer length of some tags in the display pane means that the edit pane on the right is pushed partly off the screen. Why not put edit below display or even merge them, allowing edit in place? In addition, some of the fields (particularly Title) in the edit box are far too short and, for some reason, the edit box is fixed width but the display box is variable width, which I feel is the opposite of what is needed.
That is the big issue; the only other major problem is that auto=numbering (e.g. of filenames) doesn't support a leading zero (01,02,...09,10...), which means that the files don't sort properly in Finder and I have to use Name Mangler to rename them again (!)
A decent first try, but I regret to say that there are better freeware packages for other platforms (MP3Tag for Windows, for one).