I've gone through the Harvard Extension School program. It's good. It's tough. There can be a wonderful sense of community, but you have to actively create it. One of the fundamental good aspects is the fact that there is no grading on a curve, so if everyone does well, everyone does well -- almost all profs give grade scales up front, and some even give you different ways of choosing how you want your exams weighted (in your favor).
The quality of the courses is excellent, which is not to say that I was never totally annoyed with a prof or TF, I often was, but it is definitely an excellent education. If you want to ask me any specific questions, feel free.
One word of warning -- if you don't yet live in Boston, be prepared to pay a lot in rent... I've been here a while, but a lot of my friends were pretty shocked when they got here. The courses + books, etc cost about $900 each (tuition is around $770). But Harvard is Harvard, and there are lots of public lectures -- it's a great place to do premed.