Alta, Arc'teryx, Black Diamond
What he said.
I use McDavid sliding pads UNDER my pants, and they work great for me. My BDU pants have a cordura knee, so I'm not worried about holes. They stay put, are comfortable, and my pants can move over them as needed. They are very low-profile, and are comfortable enough to wear all day. They can be had at Sports Authority for around $20.
Altas are good, but slide around, and my pant legs get bunched up.
Tried on the Arc'teryx, and liked them, but I've never worn them in any operational capacity. Heard good things.
Never tried the Black Diamond pads, but they look very well put together.
Can be a tad pricey at 90 bones for brand new ones.
I've never seen them priced that high. $60 is the average.
As for the Bijan's, I was issued a set, and I never use them anymore. They are very bulky, and very hot due to the amount of coverage they offer. I really don't care for the neoprene sleeve that goes around the back of your leg. They limit mobility too much for all-day wear. I'm sure they have their time and place, but I have yet to find it.