Pros
In my 6+ years at startups, this is by far some of the most innovative technology I have ever seen, and the best company I have worked for to date. Lots of exciting opportunities as they continue to break into (and in a sense, create the market) for emotion-enabled tech. Also quite a few women in management, so when they say "we are a flexible startup" they actually mean it - as long as you get the work done and be accountable. On a personal note, they hired me when I was 6 months pregnant, knowing I had to take maternity leave very soon. Not many organizations would take that kind of chance, or make that statement that they actually do care about employees' family lives!
Cons
For me, it was the inaccessible location (Waltham - blech), but they are definitely re-locating to downtown Boston soon!