caniuse.com provides broad outlines, quirksmode is much more specific and PPK hasn't really worked on the new non-mobile stuff so far, as far as I've seen. Nothing on flexbox for instance, although he does have box-sizing: padding-box.

caniuse is good for "is there a chance this will work?" or "could I use $some_feature in my new project?", quirksmode is "why doesn't my use of $some_feature work in $pile_of_shit_browser?!?!?!!!"

For simple lookups, pretty much in my experience, especially as it doesn't seem to have seen much by way of updates since IE9.

Though quirksmode does go into more detailed analysis in places which can be pretty useful, particularly for when you are stuck supporting legacy browsers, so caniuse isn't a complete drop-in replacement.

Yes