Pros
Work environment is outstanding. Everyone is working together to accomplish common goals and it shows. No games, empire building or hidden agendas. Plenty of opportunity to work on new technology or new functional areas. Team is still small enough that everything everyone does matters. Work is hard sometimes, but no unrealistic expectations to be 'always on' or connected. Work life is very good as are the benefits. A nice surprise for small, start up environment.
Cons
Direction changes every couple months as new partners or opportunities emerge. All the upside is in your stock options so there are no annual raises, overly generous bonuses, maturing stock grants, etc. Until growth picks up, anything other than lateral career growth is non-existent.