A.J. Jacobs is one of my favorite nonfiction writers. He's both funny and insightful, in all of the experiments he puts himself through (not just in this book, but also in The Know-It-All and The Year of Living Biblicly). I didn't like The Guinea Pig Diaries quite as much as his other two books, but I imagine it has more to do with the fact that this is a collection of articles he's written over the years, while I like long novels more than articles. But I love that he pursues a subject by actually going in-depth and trying to really understand it.
My favorite experiment is a tie between when he outsources his life to an Indian company, who write his e-mails, order things for him online, and even argue with his wife for him, and the experiment where he not only had to do everything his wife tells him to for a month but he also tries to do all the household things that she would take care of.