Loved this, only read the main trilogy so far, though. Are the others any good?
They're good, but not
as good as the original trilogy imho. There's a large time gap between the creation of the originals and the prequels/sequels and it shows. The writing style and quality is quite different and I found them a bit disappointing. They do round things out nicely though, adding more details, background and closure. Maybe try a free test run with a copy from a local library and see if you like it.
Have you tried his Robot series? Being a collection of mostly short stories/novels, its nowhere near as smoothly flowing and complex a world/story as Foundation, but several of the characters are quite notable and the moral dilemmas, though a little simplistic, are delicious.