Darden certainly has a reputation for working hard. On the other hand, they had absolutely the more engaged and positive alumni base (in San Francisco) of any school that I visited with. They were able to bring together, if I recall, about 20 alumni to meet with 30 perspective students and all seemed to really value their experience.
This was in stark contrast to say, UCLA which was only able to must 3 alumni (one who showed up late) to meet a group of 100 applicants. Maybe UCLA gets plenty of applications from the Bay Area no matter what they do (I certainly applied anyways), but it really felt like they didn't give a damn at the info session.
I dropped MIT from my list because I was scared that it might be really demanding. I don't know if this is true, or if it is just an idea I formed on my own.
I have heard that the pre-term session at Wharton is killer for those with suspect math skills. Of course, this isn't for a grade, but they work you to death.