Foundation and Empire is a book that's hard to rate, given that half of it is pretty boring, while half of it is pretty good.

The General, with its numerous easy-to-confuse characters that aren't really that interesting, along with it's lack of suspense, is a pretty uninteresting story. The twist at the end isn't as memorable as the other stories before it in the Foundation series. Finally, the fact that the twist

Spoiler

involves the characters doing nothing so that they would win no matter what

is a really bad idea for a story.
2-2.5 stars.

The Mule, on the other hand, is a much more interesting read. While there are still too many protagonists that are easily confused, the villain is able to unite all of them to become an interesting story. Combine a good villain with greater suspense as the Sheldon plan is on the brink of destruction (getting past the inevitable-win aspect of the plan that hurt the previous story), and we have a story that's, in my opinion, the best so far in the series. A cliff-hanger ending makes me that much more eager to read Second Foundation.
4-4.5 stars.

Overall: 4 stars.