Pros
People here are so damn smart. If you enjoy intelligent (although it may be quite nerdy) discussions, this place is for you. People here are also very friendly if you find the right people using the house system implemented here. Instead of normal dorms, Caltech students are divided into houses. People fortunately have a tendency to end up where they belong. You may not always feel like you fit in there at first but there is a good chance you will end up at a house that you belong it even if you don't know it. This lets everybody to find this school feel comfortable (socially of course. academically? now that's a completely different story)
Cons
The workload is pretty intense. Also people here are so smart that you won't be able to help but feel dumb at times. However, this I find is a problem you will never get away from if you want to end up in a challenging but rewarding environments (and if you're not looking for such an environment, you should not come here. trust me you will not be able to find a single review that says that this place is relaxing and easy-going)