Pros
All the requisite start-uppy type things are there: free lunch, ping pong, beer, etc. If that gets you out of bed in the morning, then you're set. For the most part, the people are wonderful. This is a fine place to work if you want to clock in, keep your head down, and cash your (paltry, unless you're on the product side) check.
Cons
You are not working for a start-up. You are working for PayPal. Whatever comes to your mind when you think of PayPal is probably correct. You're not innovating with sexy tech. The startup bones are still there in the form of completely unqualified management. Early hires are managing large departments and they shouldn't be. There are a lot of elephants in the room. Some teams are happy, others are in a constant state of anxiety. The anxiety isn't because they're too busy. Very distracting open office that is somehow used as a selling point. If you're considering a role here, be aware that the flowery speeches in the interview haven't been revised in awhile. The company isn't "metrics driven" in that you will have no fair warning that you're being judged by metrics, but you sure are. Career advancement is mostly about luck and lateral moves. I think they finally stopped calling it a 'flat structure' but they were using the term for far longer than it was appropriate.