Well, I'll be there from 3/15 - 3/22 and I've been warned that the crowds will be pretty heavy, but not quite as bad as the week before and after Easter.
I'll be in Disney from March 17th to 20th, and it will be busy, but not nearly as awful as Easter! If you look at the MK hours, the park will only be open until 10pm, which is a fairly good sign, since they keep it open until 12 when they expect major crowds. I also booked my room about a month ago, and there were rooms still available.
That being said, there are a number of colleges out on Spring Break that week, so I am planning to go at opening every day (to avoid the spring breakers who sleep late!). I went a few years ago at this time, and it was busy but nothing that careful planning and FPs couldn't help. I think we waited for about 20 minutes at most, although we did try to go to MK at 7pm one night and waited 45 minutes for Peter Pan! The park was definitely full of spring breakers in the evening.