We had a student transfer into my class (he started second year with us), and he said that it really wasn't that difficult at all - I think it's all about how willing the receiving school is to work with you. This year, I think there are 2-3 students transferring in to the upcoming 2nd year class. Though, each class did lose 2-3 people for a variety of reasons - I think this is why they were more willing.
Like was said above, often the issue is if the curriculum match up, otherwise you have to take extra classes, which sometimes you simply can't fit into your schedule.