As it's been said in the comments, it's amazing that after years of lobbying IE to change its box model to match W3C, we realize that maybe IE's model made more sense after all.
Not really. Everybody I've known (in about a decade) has felt the IE box model made more sense.
But here's the thing: it wasn't the standard, the standard was not going to change its default box model, and dealing with a single slightly worse box model would always be better than dealing with two completely different box models.
box-sizing is the alternative, and a pretty good one. But I don't think anyone ever read about the differences between the W3C box model and the MSIE box model and thought "wow, the W3C box model makes so much more sense".
Glad to see others think this too, I always thought it was just me.
That actually seems the wrong way round to me. If you think about a document, the margin is what separates text from the edge of the page (or interior padding, if you will). Padding, to me, implies an area around an element that other elements can't touch.
Even if the majority of people preferred the IE5.5 box model, it didn't follow the spec and it is therefore wrong. We have standards specifications on the web to maintain cross-browser compatibility; if browser vendors don't follow them then whats the point?
Specs should standardize what's generally considered a good idea. They are not written in stone and have no value in and on itself. If IE's old model was consensually better (and I'm not sure it was), the spec should have changed, not the other way around.
After all these years writing CSS, I'm very biased and it's hard to evaluate which is better. I was aware of border-box, but I thought it was Webkit only. Though I probably won't make the switch altogether, I'm glad to know we have a choice.