I find id3xc a GREAT product if you're in need of changing tags on mp3 files.
It has a good deal of built in functions pre-programmed and most of your needs will probably be satisfied by using these functions.
If you are inclined toward programming, the built-in id3-editing programming language allows complete flexibility over how id3 tags of a group of mp3 files can be changed - if you can imagine it, you can probably do it.
My wish list for future versions of id3x includes a couple of items: First, I'd like id3x to have some recursive functionality - eg carry out the task on all files in subdirectories of an entire tree as opposed to just a single folder. Second, I'm finding that I'm making a major commitment to the new AAC encoding and I would like to be able to edit m4a tags as well.