The best place to check statistics regarding average gpa, mcat, and acceptance rate on the school would be the US News and World Report (I don't take their rankings seriously, but their statistics are hard facts). I went to medical school in Manhattan, although not Mt. Sinai. Mt. Sinai is in a wonderful location in the upper east side, very clean and not too expensive (relatively).
I would have to say reputation wise they are really well respected in New York and the East coast. Nationally they are not as well known. I think the general perception is that for medical schools in Manhattan, Mt. Sinai is considered a notch below Columbia and Cornell, roughly equivalent to NYU (although NYU is more recognized nationally), and a notch above Einstein.