oops. I just re-read the original post and realized the poster was not talking about the basic premed sequence (I think). My comments below are only for the basic sequence: chem, orgo, bio, and physics, as that's all I took. I actually have no idea what the upper-level molecular or cell bio classes are like, or anything else. I thought about just deleting my post, but figured it might have some value to others who might be interested in the general topic.
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I'm a former Harvard student who took classes at the extension school, and I have to disagree. I think there is a significant difference between ... say Chem 17/27 or 20/30 (the undergrad orgo classes), and the orgo classes I took at the extension school. I know because I tried to use some of the undergrad notes to study for my extension school class, and the undergrad classes required a significantly deeper comprehension of and facility with the concepts.
Likewise with physics ... my extension school physics class had students in it who did not remember how to do basic algebra and required a lot of extra math attention, which you would never find in the undergrad. And with bio, I only attended 10% of my bio lectures, and still did well in the class, which again I'm sure would have been much more difficult in undergrad. My three friends who have done the same (attended the undergrad and later taken the basic sequence of classes at the extension school) all pretty much concur. So to the original poster -- I definitely wouldn't worry about anything being over your head.
I actually think it's quite advantageous that they're easier. I found that the extension school classes I took covered enough material for me to do well on the MCAT. I rarely had to devote very much time to them (~5 hrs/wk/class outside of classtime), the grading is quite generous (compared to the undergrad), the classes are very cheap, and the professors are very willing to write recommendations (which was much more intimidating in undergrad).
I tend to think it doesn't really matter where you do your post-bacc ... most med schools will not weight the post-bacc as heavily as your undergrad (unless you're substantially older). As a previous poster said, I don't think they're going to equate Harvard Extension with Harvard College, but it still has a solid reputation as a post-bacc program and I think most adcoms will recognize that it's a great program.