forum crashed when I posted and I lost my message...
Anyhow, the jist of what I had typed was I use Joe Rocket Alter Ego pants, and found the knee protection to be completely useless. I removed them and replaced them with a pair of Dainese Knee V Guards, and couldn't be happier with them (GP Sells them for around $80 --
http://gpbikes.com/dainese-knee-v.html).
The important thing is to get ones that have the strap above the knee -- Originally I picked up a pair of Icon Strykers which didn't, and promptly took them back for an exchange (since they didn't articulate at all.
Also as others have mentioned outside of pants a back protector might be a good idea -- I removed the foam pad in my jacket and bought an AlpineStars Bionic back protector instead.