Well, for full disclosure, I didn't turn down any top-caliber schools to go to MIT. But knowing what I know now, I wouldn't have chosen to go anywhere else.
I love the atmosphere at MIT, both socially and academically. I loved that I was able to push myself to learn and do and grow, but that I never felt that other students were competing with me -- I valued the collaborative atmosphere. I was able to take hard classes and do cutting-edge research, and I developed more as a scientist than I would have if I'd been somewhere that didn't value the intelligence of undergrads so much.
I really loved the freedom given to students in many aspects of MIT life. Students choose their living groups and the way they want to spend their dining money. Students can choose to take graduate-level classes and/or classes for which they don't have the prereqs.
I'll try not to wax poetic any longer, but I did write
this blog entry on my feelings on MIT students as a whole.