The Amber series was fantastic, well worth reading all ten books, though I agree with others that Corwin's books were better.
As for John Betancourt's prequels, they were spoiled for me to a great extent by the shoddy editing/proof reading. The last in the series had more typos and words/sentences out of order than any book I've ever read. The stories themselves aren't bad, Betancourt has some nice touches and he's learned the Amber canon very well. But you know that Zelazny would have created something an order of magnitude greater had he written the books.
As for whether the books should have been written, this is a subject that has been done to death in various newsgroups and mailing lists. In dedicated forums, opinion was split down the middle, but a lot of people who were initially against did eventually read them (and some even admitted liking them). I think they are worth a read, but they're not essential. There was going to be a second prequel trilogy but
AFAIK it's on hold due to the collapse of ibooks, who published the first.