I had a Horizon Perfekt, it did not leak light or anything like that. The lens I found to be pretty sharp, in fact, it was very sharp. Obviously if you don't get the horizon straight, it'll look very curved.
I liked it on balance, it was a cool camera, no reliability issues, easy to use etc. It goes without saying that the XPan is probably the better camera, but it's also priced accordingly. I think the Horizon is actually a pretty nice camera.