It means that a 300px wide image won't fit inside a 300px wide box any more.

You need to decide whether your sizing for each element depends on your layout or your content, and choose accordingly. If one way is more useful to you nearly all of the time, then use it that way.

However, also be aware that the star rule is the least performant rule in CSS (http://stackoverflow.com/questions/1714096/why-is-the-css-st...).

Not according to Paul Irish: http://paulirish.com/2012/box-sizing-border-box-ftw/

The biggest downside is that its only supported in IE8+

We use it for the Chartbeat dashboard (demo here: http://chartbeat.com/demo/) and it's been delightful, but we also made the decision while we were already doing a redesign. This meant we weren't just adding another rule to the page which would change everything; we wrote new CSS with this concept in mind.

1. The * selector is slow

2. No IE < 8 support