Big opportunities for ambitious people, fun culture.
Pros
People are really nice and fun, they provide free lunch and snacks, and it's a pretty open place. We have a monthly lunch with the CEO where we can ask anything and learn about what is going on in all areas of the company. That transparency is nice, as some companies hide the interesting stuff from everyone but senior management. Management (including but not only CEO) seems pretty accessible for pretty much anything. There are definitely opportunities to move up and take on more responsibility for good performers (there are several examples of people who made huge leaps in their career progression). Promotion and opportunity is tied to ability and hunger, not tenure, length of experience, or politics (that I've seen so far). Compensation and benefits are pretty generous. Face time is not an issue as long as you get your work done.
Cons
The office location (Newton Center) can be annoying if you live in Boston/Cambridge, and the existing office setup doesn't feel like a tech startup. Green line is a workable option for commuters but can be slow. We have plans to move later this year and management is conscious of it, so hopefully that will improve. I heard the company was moving in too many different directions before I joined (last year), but I haven't really experienced that. Management seems to have a better plan for where the company is going, but we'll have to see if that continues. Maybe it's just normal growing pains in a startup.