Hmm.. I guess it depends on what you mean by "I had an extremely unpleasant time." Did they drop the ball on processing your application? Were they mean to you during a visit/interview? Was the problem with one administrator or faculty member or the school in general?
I would think a school would want to know about administrative mistakes and errors, and that would be a straightforward complaint. A specific complaint against a rude faculty member would be another story, and could burn bridges. Whatever it is, maybe wait a couple of weeks to mull it over. Can't hurt.
I felt kind of pissed off at one school's application process and have thought about saying something.. It's hard to feel like you wasted money and effort.