There's nothing more unreliable (in my opinion) as far as a casting medium, than great stuff. If you do try it, make sure you mist water into the mold as you spray in the great stuff, it greatly helps it's expansion and curing.
Two part urethane foam with out a doubt is better, but not cheap either. I plan on trying that this year too. I am sure the great stuff skull would be good enough for viewing from a distance and with a little creative painting.
Besides I got three cans left over from last year, might as well give it a shot.
Though it is more expensive for the two part you get roughly 40-50 skulls out of it for 50 bucks. That to me is worth it since ive been paying 6 bucks a skull and cant carve anything into it. It sets up easily and works well.