Pros
PEOPLE: Culture is always top-of-mind and that nets a comfortable environment that's ripe for experimentation. People are friendly, generous with their time, and well-versed in their area(s) of expertise. Have a question? Ask. Want to try something new? You'll get pointed in the right direction. Everyone is ready to help you do things you've never done before, which keeps the day-to-day fresh.
PLACES: Being smack dab in the middle of Downtown Crossing, in a wide-open, collaborative office makes getting to (and doing) work 1000% easier. Most people have the opportunity to work from wherever and whenever is most productive and comfortable (avoid the snow-pocalypse, stay in your PJs). The work-life balance is first-class.
THINGS: All the standard tech company baubles are there (free food, happy hour on Fridays, etc.). But nobody mistakes the icing for the cake. You get the opportunity to have fun and get to know your colleagues, but there's no mandatory fun.
Cons
The only potential cons are common in any high-growth company: some of the benefits are still being worked out (e.g. 401K match, as others have pointed out), and you have be ready to move quickly and iterate. Basically: you have to know what you're signing up for.
