I actually did a 4.5 year program for Master's and Undergrad there, so I don't have the typical experience.
I liked the program, I definitely learned a lot from it, although I can't honestly say I've used it much in my career (ended up lower level than UI often times).
My only gripe is that I wish that (at least at CMU) they had taught more graphic design. We had a few classes, but they were quite basic (I took as many as I could, but ultimately the interesting ones were restricted to Design majors).
Before you go get a Master's -- think about which part of HCI are you most interested in? Usability (evaluating, running tests), UI design, the programming?