Pros
Some of the smartest, most dedicated coworkers I've ever had. Tight, focused teams tackling difficult technical projects for high-profile clients. Good pay. Young, growing company. Lots of fun "perks" like kegerators in the break room, regular company-wide social events, etc. Generally young and progressive, with a commitment to diversity (at least in individual contributor roles) and support for LGBTQ employees.
Cons
You are going to work as hard as you ever worked in your life and you might not feel like it's good enough. You will be told over and over again that there are no problems with overworked employees and yet most people around you will be working 60-80 hour weeks on a regular basis – this is neither normal nor good. All those "perks" are, in part, clever ways to make employees forget that they're devoting their entire lives to the company. You don't need to go out for a beer; it's here in the office. You can have a thriving social life, just do it here in the office or at events with coworkers. While I was there, on the same day a UX designer talked about working 40 hours a week at home – so he could get his work done outside the other 40 hours a week he was in the office - the CTO, with an entirely straight face, told me there were no problems with overworked UX designers. WillowTree often sets up engineering teams for failure. My own projects, which faced external deadlines, were defined late, started late, understaffed, and rushed. This is in large part because of the push to sell add-on business to high-profile clients (Fox News, HBO) requiring untested technologies on timelines with zero slack. Management often acted like a bunch of unruly frat boys, cliquish and oblivious to the damage they caused. Promotions often seem decided in advance based on who best fits the Bro Culture. While individual contributor positions feature a good bit of diversity, the leadership team does not. To be blunt: of 13 leaders featured on the web site at this time, 4 are women, and there's only 1 non-white face in the bunch. Candidates are often weeded out of the hiring process due to lack of a "cultural fit." If you are interviewing, and start to think you won't fit in well with your coworkers or management, you should probably trust that instinct.