When used properly Great Stuff expanding foam is an awesome substance.  it sticks to pretty much anything, expands into gaps, and is water-proof.  Its designed to be used to insulate in little gaps and cracks that are two small for regular types of insulation.   Although it probably has a decent R-Value, its far to expensive to be used in any significant thickness, and is much more useful at preventing air leaks.   I used it on this outside wall to make sure that all the gaps along the bottom of the sill plate were plugged (some bugs had previously found that they weren’t)  as well as to seal around the sheathing where holes were cut for lights.  Ohh and if you’ve ever gotten this on yourself,  a little acetone before it dries takes it right off.  Just don’t let it dry because once it does its practically impossible to remove.