Ex-Google SWE L5 (Senior) - Left in 2014 after almost 9 years spanning from my late 20's to mid 30's.
I really can't complain, and would easily rank it as my most fulfilling and lucrative full-time employment experience so far, though the previous and post employment were in defense and startups.
In short, I left due to burnout, though I think it wasn't so much the team/work as it was my character lends itself to burnout if I'm not very careful to erect work/life barriers and not trample them in spite of myself. I also tried hard but failed to get promotion to L6 SWE, and that left me with a bad taste in my mouth. This was mostly a matter of personal immaturity at the time though. In retrospect my technical skills may have passed muster, but my ability to make things happen in the organizational and interpersonal sphere weren't really at an L6 level. My foot was already half out of the door by then, anyway.
In the intervening time, I've worked on an (unsuccessful) Android game, worked at a startup (again, burnout is easy when you internalize the existential precariousness of this sort of venture), and have since moved back to my hometown, bought a nice house, started a family, and done remote contract work. I have a few projects incubating and am planning on pursuing some entrepreneurial bootstrapping once my current contracts peter out shortly.
I miss the proximity to amazing engineers and casual availability of supercomputing resources, but in the end, I'm grateful to have saved enough to have a great deal of freedom in how I spend my days and to have been fully present for the first few months of my son's life.
Another benefit of time outside of Google is getting acquainted with the equivalent software ecosystem outside of their walled garden. Borg -> Docker/Kubernetes, MapReduce/Millwheel -> Spark, Dremel/bigquery-> Presto, etc etc.