I think I'm in the consensus that illustrations can be great if they match the subject well. Some of it depends on the target audience; for children's books, for example, they are virtually a necessity, while YA can benefit from them and adult could go either way. I think genre is also pretty relevant; illustrations seem to work better in fantasy, while schematics seem to make more sense in SF. Maps are genre specific as well; they seem to be very helpful in fantasy, but I can't see a space opera benefitting much from them.
I think the subject of the illustrations can be important and vary with genre a bit as well. I like having illustrations of the races featured in fantasy, but I am less interested in this in SF as I am interested in pictures of the tech in use, such as the protagonist's ship preparing to dock at a spaceport. Landscape shots can be quite useful in just about any genre, though.