This really wasn't that bad - in fact I enjoyed it quite a lot - but it was very disappointing. It is an entirely different kind of book to Foundation, which was about concepts. Not amazingly written, certainly, but neither was this, and without the great concepts, there's not a huge amount left.
I think it would be a bit harsh to say that this book was written to cash in on the phenomenon that was Foundation, though I suspect that is part of it. What probably happened is that Asimov realised that he could link up all the books he'd previously written (for the Foundation series and for his robot stuff) into one big series, spanning thousands of years and the entire galaxy, but still essentially linked. Which means he had to write a few books to go in between the early robot stories and the later Foundation series. Prequals to the latter or sequels to the former? It doesn't really matter, because all this book is (and I expect the other prequal/sequel too) is a gap-filler.
So this book draws out connections and follows its plot in an entirely arbitrary yet painfully predictable way. Unlike Foundation - which was delightful because it hardly paused for a second on any particular group of characters, instead focusing on broad sweeping principles of politics and economics - Prelude to Foundation follws, tortuously, the path of Hari Seldon in his quest to develop the science of psychohistory. This involves close character study, something which Asimov is very bad at. Also, while he's pretty good at the political/economic stuff, he's appallingly bad at the anthropological side of things. The definite low point of the book was the sojourn in the Mycogen sector of Trantor, during which I spent most of the time feeling both appalled and insulted.
The real thing that made Foundation great was that it left so much unsaid - it treated the reader intelligently, allowing them to make their own connections, instead of explaining every minute detail of a plot development whose existence any observant person would have guessed fifty or so pages earlier.