Pros
The greatest thing to work at Harvard University, to me, is to share the best ideas with most talented people. sometime I even think that I am so lucky to get or "steal" a lot of great ideas from other fellows at Harvard University. In a wider range, there are a lot of great schools around Boston area, and it is absolutely a great thing to enjoy your research in an environment of great schools. It is also good for career once you have experiences at Harvard University, which is a very big plus--agreed by most people in my field.
Cons
There is no obvious downsides of working to me at Harvard University. Some people told me that the atmosphere was a bit tight at Harvard, and people felt a bit (or maybe more) pressure here. Well, it is hard to say, should be case by case. To me, my boss is so nice to his fellows, and the working environment is good as what I described above. Some said Harvard professors gave less salary to his fellows because most people would accept the low offer to get working into Harvard. Basically I agree, but it is just like a market and your investment... you are still the one who decides go for it or not.