I noticed the thing about the resistance and the knees, and I think getting the crossramp too high has affected them too. I've never had knee problems but they did get a little clicky - nothing bad and no real pain once I got off the machine. I've started to do one other other other - lower crossramp with higher resistance and lower resistance with higher crossramp. I can still get my HR high doing this.
It's hard for me to evaluate my condition while using it, though, because it doesn't make me tired or heavily worked when I get off. Yesterday I did 7 miles on it at about 160 bpm almost the whole time, and didn't feel bad - in fact, by the end of the hour, I felt energized. Because of my injuries, the most I was managing on the treadmill was 4 miles. It made me wonder, though, because you would NEVER make a jump like that with running. 10% at most, so if I did 4 mi last week I'd be doing 4.4 this week. But today, my hips are just a little sore - I mean a TINY bit sore, barely at all.