Pros
Business is doing well, seems to be in the right place at the right time. High growth company (expanding rapidly in all dimensions), free food, fun environment. Charismatic CEO gets attention and buzz for the company. New VP of Engineering bringing maturity and leadership to engineering organization.
Cons
If you're industry experienced (not "senior" as that's 1-2 yr out of college at Box) expect to be working almost entirely with inexperienced/young engineers. Most are great to work with and want to learn however in general the culture does not value experience; be prepared to see lots of questionable judgement/competence demonstrated by the homegrown tenured developers leading projects. Engineering caliber and quality of people is below Google / Microsoft levels. Surprising number of "seniors" lacking CS fundamentals (OO design, what's cohesion? using assertions?) Some B's hiring C's going on (... will hopefully improve as Box adopts more google like model in hiring, reviews, etc.) Lots of untested legacy code, 10,000 line functions, tightly coupled and fragile code is common place. Major efforts to write tests and turn quality around but very much work in progress.