Why does the entire web community agree that feature detection is the way to go and UA sniffing is bad, but time after time shows that Google does it on nearly all of their products?
Compatibility view doesn't just change the UA string. It actually switches to a different (IE8-like?) engine.
It changes to one of: IE7 Standards Mode, IE8 Standards Mode, IE9 Standards Mode, IE9 Almost-Standards Mode, IE10 Standards Mode, or IE10 Almost-Standards Mode. I wish I was joking.