Yeah dwig I'm with you on that though sometimes it is the most budget way to get the books. I have other reasons for splitting them (besides ease of calibre metadata!) - it makes it an awful lot easier to keep track of books read (via Goodreads & my plugin for it), since it is frequently the case that I don't want to read all the books contained consecutively.
I agree with you delta888 that it isn't a trivial exercise though if you have a lot of them, so since you have a plan to split them eventually I would suggest just picking a temporary workaround that takes your fancy and run with it. They do create all sorts of thorny problems - some like dwig mentions, or others like the complications of where the omnibus contains "random" books from the series rather than sequential ones in reading order, or where it contains the whole series (so then do you bother showing it as a "series" at all)... ugh.
