The best part is the calibre of the people I get to work with. It's incredible to work in a place where, in every conversation or meeting, you can count on the people around you to individually and collectively be expert -- and I don't mean in the sense of self-proclaimed "thought leader" expertise, I mean the legitimate kind that's humble in the face of the the enormity of what one person can never learn. That's common at Google -- in all job roles -- and it's incredible.
The worst part? It's a big company. Big companies are worlds unto themselves and getting important things done is orders of magnitude more effort than it seems from the outside. But the pay's good, so it's hard to complain = )