I've found this stuff to carve very well, once you get the foil off (which is a horrible pain in the...patience).
I've carved most of a skeleton out of this stuff - ribs are tough, but I think that'd be true with any foam.
Finger bones are easy, and you just slide them over some wire to make hands - dust some spray paint on them (unlike styrofoam, this foam doesn't dissolve with spray paint solvents), and you have modifiable, weatherproof bones.
Beware on long bones - this stuff doesn't have a lot of strength once it's been thinned. Run some heavier wire down the center, and that will help the bones hold up to mild to moderate abuse.
Of course, you DO need to get the foil off, and for that, I wish I had a simple solution.
Edited to add: polyurethane glue works fantastically on this type of foam (almost as if it were made to do so), and since cured polyurethane glue is carve-able, you can make large slabs and blocks of this foam by laminating sheets together.