Not everyone will like this book; in the same way as not everyone will like (or, more simply, have time for) Joyce or Proust. I don't mean that Wolfe writes like either of these. But rather, in the particular work (and others) he needs patience, the right mood, and the right expectations before you will get what he is trying to tell you (or, confusingly, not tell you! :).
I generally dislike writers who whose works aim simply to manipulate the imaginations of the reader for no particular purpose - for example "deconstructionalists" and the rest of the postmodernists whose goal appears to be to demonstrate their own cleverness at the expense of producing anything readable or entertaining. In "The Fifth Head", Wolfe takes one idea from that school - namely, that you can tell a story only by hinting at it - and turns it into magic, while at the same time never insulting the readers intelligence.
I confess I've never enjoyed any book that has attempted something like this, before "The Fifth Head of Cerberus". When you have read all three novellas, you realise - slowly - that there is another, internal work that is both parallel to, and in contradiction to, the written words. It's hard to explain, and surely a hundred times harder to write.
To those who didn't enjoy the work on the first reading, I would say to wait a couple of years and try it again. It is one of the most rewarding works in SF or in any genre that I have read, and it deserves the deepest reflection.