I guess it depends on how flexible you are. Eddie Bravo is a very wiry 150lbs. I haven't seen any big or stiff guys use it effectivly. I am pretty flexible so it enables my guard to be pretty dangerous. I have worked the rubber guard and find it useful to work into omoplatas and to keep my opponent from posturing when I go no-gi. I don't feel any stress on my knees but I am not holding the rubber guard for a very long time either.http://trapacademy.com
^that guy up there is right, depends on your flexibility^
me, i am flexible cause i use to take tae kwon do..
if you don't have any flexibility, consider taking yoga or tae kwon do for a bit..
you probably feel pain on your knees cause your not flexible on your leg and hip area.
good luck with training
I've got a few students who are smaller in stature and have great hip flexibility. They use the rubber guard all the time w/ good success. But neither I or any of the bigger guys I train can pull it off. Just too tight in the hips.