I just got PDFtoEpub for free here:
http://www.pdftoepub.com/authorspromotion.asp and so far it has done very well converting some books I had that even Mobipocket Reader/Creator couldn't handle. The resulting epubs may still need a few corrections in Sigil, however, to fix things like spacing or the infamous "missing" apostrophes, and on occasion, the apostrophes that somehow get switched to reversed quotation marks but to be fair, many programs do that when converting .pdf files, not just this one. So far I like it, you may want to give it a try.