No, it's not legal in muzzy season. Although since i have a license to use a scope in muzzy season I probably could use it. It's legal in rifle season for everybody. (I think)
I did try one. It's pretty weird, but the sight was in focus for me. That part was nice. It would be better than an open sight as both block the bottom of the target, but the SeeAll moves with your head, so you don't need to be centered on the stock. Which makes taking quick shots better. You also don't use a front sight, so nothing to line up. Just put the SeeAll reticle on the target and squeeze.
With all that said. It's not as good as a peep.