Pros
First and foremost I would say that on the whole, most of the people I worked with were smart, friendly, and very motivated individuals. When I had a question about something specific to the code base, I didn't have to wait very long to get a response back. Moreover, most of the code base is fairly well documented on an internal wiki. My manager was a great guy and gave me tons of positive feedback and encouraged me to explore all the unique opportunities Expedia had to offer. It is also worth noting that there are numerous opportunities for self-growth offered by the company, like language classes, that can be completed on your own time or sometimes even be counted towards your current work initiative.
Cons
Similar to most large code bases, there is a significant learning curve in regards to domain specific knowledge that needs to be overcome before one is able to start making useful contributions to company. Additionally, there was quite a bit of downtime while I waited for my computer to be set up which further set me back from making progress. I actually joined the company in the midst of a new site release and consequently I wasn't able to start working on the main code base until the release had executed successfully. When I worked at Expedia we had offices, but the company has since relocated to a new building with cubicles which I am less fond of.