I have used the Giro Air Attack for a couple of years now. It gives you less wind protection than proper cycling glasses, but once I got used to it I started to hate putting cycling glasses on as it feels more sweaty around the eyes.
I usually use contact lenses, but have used it once or twice with prescription glasses which was fine.
Never had any steaming up problems.
Flipping the visor seems straightforward to me, but it's not something I do more than once or twice on a ride.
I think for UK conditions they are pretty much perfect. Bit warmer than most helmets, which I prefer except in mid summer.
The main advantage is the rain protection. Keeps your eyes and head very dry and you can still look through the little air gap of the visor even when the visor is wet. Great for contact lense wearers as no water gets to your eyes at all.