Pros
I enjoyed the open and giving atmosphere. Collaboration and cooperation was definitely the norm at Zillow -- everywhere you went, there were people having impromptu meetings, sketching on whiteboards, working out problems together. Everyone was focused on creating the best product possible and invested in helping others create their best work to get there. The development cycle was tight controlled and executed, which allowed for quick turnaround on features. Everyone gave 100% and was serious about their job -- I can only think of one or two people that didn't seem to pull their weight (and there's always at least one, right?) All in all, the people were absolutely the best.
Cons
For me, I sometimes felt that, since I wasn't a java dev, that I wasn't privy to a lot of information that they were getting. They seemed to know about a secret repository of knowledge that no one else did, so I felt like I was playing catch-up all the time. I personally was not given very much training to start, so I felt there were major gaps in my knowledge about how the site worked and was put together. I was issued a VERY slow computer that impeded my productivity tremendously, and prying loose the resources to buy new equipment was difficult.