Before I went to clipless I was having frequent knee pain... Being able to pull up on the pedal and allow power to be transmitted in different ways takes some burden off your knees.

But you might forget you're wearing them, fall over and whack your that knee that's been sore; that exact thing happened to a buddy of mine...

So be just careful if you get them... I recommend learning on the "multi-release" cleats; set the pedal tension slightly higher then normal pedaling so when you panic and yank your feet up hard they release...