Pros
The engineering team is very strong technically, and a lot of fun to work with. I didn't know at first if I would like the totally open environment (no offices or cubicles), but it is surprisingly comfortable and beneficial for collaboration. The atmosphere is very relaxed and a good life/work balance is encouraged, not just given lip service.
Cons
Growing pains. This strikes me as a fairly common problem with small software companies. The processes, or lack thereof, that worked well enough in the past are now hindering growth. We are now going through the difficult process of trying out new processes and finding which ones will work best for us. At times this is painful in terms of defining requirements for projects and predicting a release date.