I don't wear knee pads anymore. I wear pants that have pads built in, like the new JT pants, the Smokin' Outlast, or Dye's C4 pants.
Here's my view:
Dye pads stay in place better than most pads. They are minimal in size, do an adequate job protecting your skin, and even provide a little bit of support for those with bad knees. The downside is that they pull out your leg hairs, make your knees smell like sweaty feet, and bunch up in the back of the knee, pinching the skin and sometimes impeding circulation to the skin there.
JT knee/shin pads are cool, comfortable, and light weight. They are basically sleeves of lyra with a pad sewed on. The lycra doesn't pinch, and it wicks sweat very well. They cover a lot of area, so they protect more flesh, but they don't have as thick padding as the Dye pads. Worst of all, they don't stay put; you will find yourself pulling them up over your knee all the time. They tend to ride just below the knee cap if left to themselves, which is sort of useless. Wierdly, the dye in the pads is soluable with the oils in paint fill, so you sometimes end up with large stains that look like massive bruises. Fortunately, these stains wash off easily. I wear these on my arms now, and I look like Popeye.
Redz pads have a built in angle that I don't care for. Also, they have a hard plastic plate that guarantees that there will be no lucky bounces from your knees. They are made of neoprene (surprize) and they have similar pinchy and sweaty characteristics to the Dye pads.
Empire Ventz are basically JT pads with somewhat thinker, bouncier padding. They slide down very similarly.
Xcel pads slide all over- like Dye pads without the main advantage of staying put.