Pros
I've been here for almost 3 years and I've learned so much. The opportunities to learn and implement bleeding edge technology is unparallelled. The open environment adds to the atmosphere and seems to encourage camaraderie among the groups that share the space. The co-workers here are the best part. No one is a douche and everyone gets along. There's not a single person here that I wouldn't drink a beer with, which is a nice segue into my next topic: Beer. More generally, the social events that WillowTree hosts in order to foster closer relationships among co-workers. Weekly beer:30's on Friday where we get a beer and just socialize. Weekly R&D meetings where we get pizza and talk about what's new in our technological world. It's something that is probably underrated by most. For a company like WillowTree to have the Esprit de corps that they have while still growing fairly rapidly is a feat in itself. The usual pros apply here as well: Good pay, they give you a Macbook to work on, 401k, profit sharing, etc. They're all nice, but nothing beats a workplace that you actually look forward to going to every weekday.
Cons
I suppose the upward mobility is fairly limited, since the management structure is fairly flat. Also the open office is nice, but can get loud. Personally I like it, and it tends to become white noise most of the time.