I use the belt all the time. I use it as both for both climbing and sitting.
I should use a harness, just my experience with a harness was every time I pulled it out of my pack it seems the thing always got tangled. Trying to untangle an all black harness in the dark is no fun. I'm sure they make them better now.
With a belt I always cinch myself in pretty tight..I don't really like being in a treestand. I don't have it like some where they can stand up, lean out with the belt holding them and take a shot. If I'm sitting I have to readjust the belt to stand up. I'm sure it cost me some shots before, but I like the snug feeling.