I am going to attend the Harvard Extension School this summer and I have been very impressed with the program and all that it has to offer.
From what I have gathered the Harvard program is much more academically rigorous and less professionally driven.
Though some of the attendee's are often professionals looking to change careers or build to their resume's, the peer class of professionals have amazing credentials and offer great connections once your study is complete.
Most people will tell you that Harvard is not simply the best school because of it's academic rigor, but that it offers the best opportunities to make connections and get further in you given profession.
The only people who discredit the program are some people who live in the Boston area because they have attended classes with auditing students (those who are not pursuing their MA but still attend through the Extension school) and have found them to be un-motivated and lack the intelligence or will to compete at Harvard level of excellence. This is somewhat true. However, it doesn't discredit those who do attend and pursue their Masters, which takes a great deal of time and rigor on par with other Harvard grad programs (despite what Law students or Business students might claim).