Rated 3 out of 5 stars

The main problem is that measureIt doesn't actually measure html elements, but rather measure how the user is drawing the ruler on screen.

While this might be useful for rough measuring, it's not even close as useful as the built in Web Developer tools in Firefox, that gives you the actual width/height, padding and margins for selected elements.
Also Westcivs Xray http://westciv.com/xray/ does a much better job than measureIt.
Both these gives you actual element sizes.

This review is for a previous version of the add-on (0.4.10).