I read a couple of the series about 15 years back and found the writing style and characterizations so dated, that I really had trouble getting anything enjoyable or insightful out of it. I feel the same amount most Golden age SciFi, with Dune being a notable exception.
He's a brilliant author, but his characters are not well developed (esp. The older stuff). If that's what you need in fiction you're probably better off reading something else.
I remember reading an interview where Asimov is asked why the books of the Foundation trilogy are so different.
The short answer is that the first book was inspired by an idea he had while talking to a professor about the rise and fall of civilizations.
The others were inspired by pressure by publishers to repeat success, and so he tried to improve and continue the story. This lead him to try and carry the story with characters, which was ironically not why people liked the first book.
The Mule was alright though, even if a little forced.
I read the first and was incredibly unimpressed. Does it get better? I love the premise, love many other sci-fi authors & series, I thought this was going to be an obviously good one.
I wouldn't have gotten nearly as much enjoyment out of the series (including the 2 sequels after the trilogy) if I hadn't read Asimov's Robot Series. I know that the whole "you don't get it because you haven't read these other books" thing is kind of cliché, but I really do think that the majority of the joy I got from Foundation was because of how it connected with the Robot Series.
But of course, part of the enjoyment should come from the book itself. I'd say that the book does get better as new threats to the Foundation appear, but the premise largely stays the same throughout the trilogy: will the Seldon Plan succeed?
(NOTE) The reading sequence for me went something like:
1. Caves of Steel
2. Naked Sun
3. I, Robot
4. Robots of Dawn
5. Foundation Trilogy
