I've used a Fog City Pro Shield for a couple of years, on a Shoei RF-900 with a clear shield, and once used to the slight distortion (more reflection from my face than distortion), it's been great. It's probably time to put in a new one, as the anti-fog properties seem to diminish over a long time (cleaning it carefully helps, but scratching it is hardly aviodable).
I've also used a Super-visor for the same couple of years. Okay, it's not pretty. Okay, cool guys call it geeky. Neither of these wildly important things keep it from doing exactly what it needs to do, which is shade your face.
If you ride in and out of shady sections of the road, especially if the sun is particularly bright, you can't see into the dark areas. It's kind of exciting, to have such faith in the condition of the road and one's fellow road users that you can just charge into those dark holes without knowing what's on or in the road. Tinted shields or sunglasses darken it so you're not taking the full brightness in the eyes riding into the sun, but they darken those shady areas, too (so I'm not sold on the self-darkening shield). Shading your eyes allows you to see into those areas better. If you don't believe me, try it yourself: as you approach a shaded section of road, shade your eyes with your hand - it'll make a huge difference in how well you can see in there. Same thing for a visor in a car.
When I first used the Super-visor, it didn't come with the little standoff kit or the ampule of surface prep. The visor was not only too low (I had to tip my head back to see from underneath it, and I couldn't get a finger up under it to wipe rain off of the shield), the stickum didn't hold it on when the visor got bumped in handling the helmet. After re-sticking a few times, the stickum really didn't stick. The amazing thing about the design of it was that even up to 80 or so, the force of the oncoming air only held it in place better. The worst thing that happened was that the goop would give up and the visor would tilt down slightly on to the shield, but it never flew off. After installing the hi-rise kit with the etchant, the thing has never come unstuck. The fellow I talked to at Super-Visor said they have helmets hanging by the visors in their office to show the strength of the attachment.
I'm going to keep using both of these things. In little part of California where I live, it rains an average of 85 inches a year, I ride year-round, and the commute is eastbound into the sun in the morning, westbound into the sun in the evening. Both products make it easier, more comfortable, and safer.
If you think the Super-visor is too ugly, check out the new Arai XD hybrid helmet with the integral visor. much cooler, but still functional.