Hardly notice it's there.
I'm not accustomed to wearing much padding, but after starting some mild single track riding down by the local creek on a mountain bike a little undersized for my height, and having grown up with Osgood Schlatters, I wanted some padding for my knees. Apparently these things are always on sale, but I think they're a solid investment regardless. They can be worn as a full on slip over the knee, or strap around the front of your pants pad since they are too bulky to fit underneath. That said, I've only used them slipped on with shorts. Plenty of padding for an occasional frame tube/bar jar or slip from the pedal, and so light I forgot I had them on until about 3/4 an hour into my ride when sweat started to make them itch a little just behind the knee, though they have a small cutout there for breathing. All in all, if you need just knee protection(haha), these seem like a great buy and I would definitely look at Six Six One for future protection wear.