I have one of those, just smaller - 11x14".
The magnets are very strong; they're more than strong enough to hold FB paper flat. They're also angled, so that they can hold the paper while also allowing light to reach it. This way you get almost borderless prints.
Unlike the magnets, the edges of the frame don't hold the paper really flat. However, the curvature, though worrying at first sight, seems to be well within the depth of focus. I've never noticed any lack of sharpness at the edges.
My only problem was that it came without the advertised non-skid base. I had to glue a sheet of foam on its back.
Overall, I'm very pleased with it.