I rode the Los Angeles-Barstow-to-Vegas dirtbike ride last week and I got these filterskins just in time for the ride. I oiled them up with NoToil oil (let them air out for 24 hours, so the alcohol can evaporate) put one on my KLX250 and kept the other two in my pack for the ride.
After day1 and 120 miles of dirt the filterskin had done its job. It was nearly completely black. I pulled it off and my UNI filter underneath looked nearly perfect. So I threw another one on and did the remaining 160 miles of the ride on day 2.
I washed out the air-filter after the ride and was surprised how clean it was. So far, they've lived up to their price-point and I'll continue to use them on each ride.
I recamend this set up to everyone. I use this size filter skin to streatch over my air box lid.6-8 in. dia X 4-7 long. I cut the air box lid so there is just the frame of the lid is left. I then streatch the filter skin over the lid and then install the lid. So all the air entering the air box is filtered. After a hard day of riding just remove the filter skin and install a clean one. The air filter never gets dirty. I oil the filter skins with the same oil as my filter. I have used them directly on the filter and that to works great.