The Pyramids are by far Egypt's best known attraction and one of the top tourist sights in the world, and as such worthy of visiting once in a lifetime. I don't think they are the best Egypt has to offer - that would be Luxor and Thebes, Abu Simbel, and the Egyptian Museum -, but still you can't go to Egypt and seriously say you haven't seen the Pyramids. Why then did you go there in the first place?

Anyway, they are very impressive, and wandering at their bases (you can go inside, but apart from crawling through a long, dirty tunnel there is nothing to see or do since grave robbers were there before you) and also seeing the wonderful Sphinx is definitely a great experience.