I've had hemp seed beers and they are really good, the toasted hemp seeds add a nice nutty character, untoasted they seem a bit toasty maybe. I'm guessing really, I've only had them in a few beers, including one in Switzerland, but with no similar but undosed beers to compare.
They're totally legal to brew, I think in the US you're just not allowed to plant the seeds. I had a hemp beer at Hobson's Choice in SF, and AFAIK Kettlehouse in Montana uses hemp seeds to make Fresh Bongwater and Olde Bongwater. I've had the Fresh, it was good, Olde was a seasonal that I missed.