I'm very impressed with the program. I have one suggestion: please add a column for the "group description" field. It has no obvious use, but that's the point: it's one of the very few basic tags that iTunes saves directly to the mp3 metadata (not just the proprietary database), and I use it to preserve my 1-5 star ratings for cross-platform use. Users could use it for just about anything (mood, free association, whatever), without disrupting anything of value. I realize there are better alternatives to iTunes, but the iPod/iPhone is pretty popular. Thank you!
You can add any column you want already in mp3tag. Just right click on the column header, choose "customize" then with popup add new column. Call it "Group Description" or whatever you want, then in the "value" enter the tag name. I think "grouping" in itunes is actually this tag name:
once entered, save, then 'tick' the box next to "Group Description" so this column shows up in your mp3tag view.