Pros
The many perks including free gourmet food, snacks galore, cultural events, speakers/authors, lots of smart people from all over the world to meet and work with. Many choices of projects to work on and relative freedom to do so. There is plenty of work to do here from improving existing systems and creating new products. Kid in a candy store if you are eager and driven to make things better. Wide range of technologies - handhelds, operating systems, HTML5, Linux, file systems, compilers.
Cons
Lots of smart people, but lots of subpar people somehow managed to get hired as well.
20% time is a myth. I suppose some people get to do that. I don't know any personally as every group I've worked with is stressed and working hard on the project they're on.
At Google it was better to be early than good. Being in the wake of a mass exodus of (mostly) young millionaires (many of whose first job was Google) and being handed their mediocre work to fix can really kill your incentive to "go further" here, especially when later senior folks are not well compensated equity-wise. Google is a big company that provides a comfy job but none of the burning desire to go that extra mile because they aren't making the effort to take you with them.
