For me it works just fine. It is about 8 inches in diameter, so it isn't huge, but adequate. I have it stuck to the side of the shower stall, so it isn't under the shower head but is very close to my face so I can see well. I just fill it up with hot water before I switch from bath spigot to shower head. When done simply turn it over (it has a hook on the bottom as well) and let it drain.
Obviously, as the water cools, the fog-free effect of the mirror being warm will diminish (as opposed to those expensive ones you hook to the plumbing that get a constant flow of hot water). However, I start my shower with shaving, and keep my showers relatively short (10 minutes or so) and have never had a problem with it cooling off too much and fogging over.
oh - and I have bumped it off the suction cup mount a few times. It crashed into the tub and bounced around a bit, but survived and doesn't seem any worse for the wear.