Pros
The culture of this company is outstanding. They respect their employees, they make coming to work fun, and they empower and trust their developers. Free Friday lunches, two 20 minute breaks, and an hour and 20 minutes for lunch are unheard of anywhere else in London. Oh yeah, all those breaks are paid too (so you really only work for six hours per day). Friday afternoon (after that delicious free lunch) is for personal/professional development that can be spent learning anything of your choosing. Then you top all that off with monthly celebrations of new employees, after-work company-paid events (like board game night!), parking reimbursement, an all expenses paid orientation trip to HQ in Virginia, and a culture of helping each other out whenever you can. It really doesn't get better than this.
Cons
I get asked this constantly in interviews and am hard pressed to find anything truly bad about the company. The only things myself and my colleagues have been able to come up with are superfluous at best, such as: there's so much free food it's hard not to eat it all, talking to anybody about your job will sound like you're bragging (because it's really that awesome), and a job this good will ultimately ruin any future jobs you will have.