Pros
For the most part, you are surrounded by bright people who constantly challenge you and help you do a little better with every project you tackle. Expedia is not the place to work if you want to coast through your days. Expedia also is just a great product - travel is fun, and it's exciting to be part of arguably the best online travel company, one that helps millions of people explore the world. Plus, it has the best rewards program of any online travel site, which should give the company a huge advantage in terms of differentiation and longevity (but only if the program is managed well and incorporated into the company-wide strategy).
Cons
Expedia's future is hazy right now, in part because people are booking directly with travel suppliers (airlines, hotels, etc.) rather than using one-stop shops like Expedia. It's also due to the fact that Expedia is having an increasingly difficult time differentiating itself in the travel market, something that, frankly, it was never good at. Contrary to common sense, you do NOT get great travel perks as an Expedia employee unless you work closely with or are part of the teams who work directly with Expedia's travel product suppliers (e.g. hoteliers, airline reps, etc.). The HR department is horrible. I had offer letters left on my desk with no discussion about my salary, I was flat-out lied to about HR policy, and I found all my HR reps to be among the most unresponsive and unsupportive I have ever encountered.