Pros
Sure, from the outside people always talk about the awesome benefits and perks of Google, and the fact that you are surrounded by one of the smartest people in the world, but it really boils down to one thing: this company really respect its employees and gives ownership to them.
There is an immense transparency at Google I have never seen at other big companies. Employees can voice their opinion on anything at Google and senior management (and boy, we do voice our opinion). The energy, atmosphere, and ownership at Google is really addictive, and I can't see myself working for any other company in the future, even if the pay is better.
I do use my 20% time and actually work on another project I feel passionately about, and when you think that we are already indulged with so many benefits and perks, they just get better and better every year.
Cons
Before I joined Google, I was a superstar engineer. Now that I joined and am surrounded by superstars, I am just "average". Sigh.