I own the printed version of book one and I own the pdfs on books one through three. Is there anything in The Complete book that is not presented in its predecessors? I mean, if you own the previous three, ist it worth it to buy the Complete book?
There's also a bit of stuff from Monte's website added in, as well as a few new pieces of artwork.
The real benefit is having it all in one place, with like items grouped together -- taken as a whole, this is by far my favorite third party collection of spells. The three originals are among the most-used books at our gaming table, so I jumped at the chance to get the compiled edition partly because it's hardcover and will last.
I resisted at first, but once I spent some time during our last session jumping from one BoEM to the other, I decided it was worth by the CBoEM. Thus far, it has been.
If you have PDF versions, an added benefit of CBoEM is having the hard copy. I like PDFs a lot, but if they're books I really like and use, getting the print version as well is good.