Yes

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.

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?!?!?!!!"

ignoring your frankly unnecessary sarcasm, I'd have to point out that some of us are held to the expectation that websites should look almost the same in older browsers. My previous employer, for example, required full support of all features in IE7, and that the product should even bear some resemblance to a website in IE6. In that situation (combined with AJAXed content making polyfills difficult), flexbox isn't usable. I'm sure I'm not the only one who has been in the situation of having to support non-bleeding-edge browsers.