Pros
Great people. The majority of people you will meet and work with will be awesome. Access to learning: there are ALWAYS opportunities to learn stuff. There are tons of seminars every day, plus clubs. Also, depending on what your status is, there are courses for free (open courseware) or for pay. I was lucky to meet President Reif once, and he was on my friend's thesis committee, and we both have positive things to say about him. He's great! smart, supportive, warm.
Cons
My advisor and I were a bad match, but by the time I figured that out, it was too late to change advisors. Also, it's somewhat difficult to change advisors anyway. My labmates started out pretty great, but then the group deteriorated over time. In particular, there were a lot of people speaking Mandarin in the lab, which is fine except that I don't speak it, which led to a very isolating experience. Also, some labmates did some unethical things, and there was effectively no negative consequences to them for their actions, even though the advisor was informed. Those events tend to lead to a demoralizing and mistrustful culture. I stopped talking to most of my labmates for the last 2 years of my PhD.