First of all, I think all the "perks" Google talks about are really just HR marketing. They have good benefits, but so do Apple, Microsoft, Oracle, and any other large company. But the others don't seem to have to talk about it all the time.
The 20% thing is s sham. Believe me, no one is working 32 hours per week on Google stuff and 8 hours per week on personal ideas. Everyone works to the bone and rarely do people actually have time to work on something outside of that.
The perks like free lunches, car washes, laundry, etc are less interesting as people get older. These are all designed to keep you in the office longer. Maybe it's cool to have your social life revolve around work when you are 22, fresh out of college. But at some point you realize you want to get away from work when you are done with work.
There is less upside to Google stock than there was before. With a $600 stock price, you really need to think if you're better off staying at Google or going elsewhere. For people who are fully vested, I'm sure Facebook looks like a great option with more upside potential.
Personal opinion: Google doesn't build great products. They are a technology company. If you are working on search or ads, that's exciting. But otherwise there are few products at Google where I would feel proud to work on.
In general I find it odd that magazines and newspapers are constantly writing about how cool it is to work at Google. I'd much rather hear this from the employees themselves, in blogs or just word of mouth.
I worked at Apple for six years and I can't stop talking about how great the experience was all around. I am modeling Posterous around what I learned there. I'm not sure I've heard the same from ex Google employees.