I get praise and appreciation, and bonuses from other teams. I think it looks good on a resume, I learned something, and it builds a connection across teams. I know the things I'm doing are helping Google as a whole.
People are always going to have different feelings, and often mixed feelings, about putting time into a side project rather than concentrating on their main thing or doing something away from the computer. For me it comes and goes: some days I want a change and sometimes I feel too busy.
How many of you who have startups would feel OK about taking time from it to spend Friday working on an open source project, even if you don't have a boss that's stopping you?