I have worn them for about ummm, probably 6 years or so. I have been playing for about 10 though. WHen I started, I never wore them, because I was a kid, and didn't feel the difference. Then, I went to a tryout, and left my shin guards at home, and borrowed another kids. He had kneesavers, and I was like "wow, these really do relieve the pressure on my knees" I went out and bought some. I will say, they do help improve endurance, especially during those long innings and games. Last year I started using them only in bullpen, and some in practice, but then I injured my knee and had to have surgery, so I'm back on them. Haven't gotten to use them much, its only been a month, but I will have to use them. I have never noticed them to be obstructive in anyway.
I used to have trouble blocking, partly because I sat all the way back on the knee savers, and partly because I'd never really known how to do it. I learned last year (yeah, kinda late, but better late than never), and I am getting better at it.
Knee savers must be used as knee savers, not as knee rests. They can't be used all the time, because you're being lazy, but if you never use them, why have them at all. There is a fine line between genius and insanity, the same goes for using knee savers.