It is far easier to paint a door whilst it is hanging, you can do both sides and the frame without having to handle the door.
I've used them and my Dad (a lifelong P&D) also used them sometimes. They're great for getting the stuff quickly and evenly over a large surface, however on things like doors we tended to 'pull-out' the paint afterwards with a good brush following grains etc.Do gloss rollers really work then? I tried one a while ago, and reverted to a brush very quickly!
Any tips or tricks?