Rubber guard is the best thing to happen to the bottom game since someone decided to fight from missionary position. It is a great way to control your opponent as it severly limits their mobility and defense. A big enough person can muscle out, but will probably give up a sub if they don't know whats going on (which is pretty standard with all guard stuff). A smaller person will have a lot of trouble getting out, and will probably just try to hide and hope for a chance to escape.
With strikes thrown into the mix it is a more challenging to use because its a little hard to defend against the arm that isn't trapped, but any strikes that are big enough will leave openings for triangles. I try to avoid the getting there by going for an omoplata as soon as I get rubber guard.
It does suck for your knees, but I found it helps if you don't use the mission control setup (pulling your own ankle across the back). Mission control work very well, but unless you are very flexible it will leave you hurting badly.