It looks up the data (as it should be), so you rarely will have to enter anything yourself. It sometimes helps to add data for 1 track for horribly mistagged stuff, but after that you can usually drag and drop the rest of the album.
MB is a bit of a monster. If you're tagging a lot of music you may want to install it locally. My experience with this is documented at http://acooke.org/cute/Installing3.html
Personally, I was not too impressed with MB itself - ended up using LastFM's API instead. But this was for generating playlists, not tagging.