No, it's doesn't really interfere with my grip, as it's on the index finger, and you usually put more pressure on the two middle fingers. Yeah I love this little device. I used my iphone before, and that was a hassle.
Yes I only measure TUL and weight. Reps are irrelevant as I do super slow movements anyway. Just hold out in pain as much as you can!
I used to train with free weights before, but that's not an option in the gyms I use now. When I finish the exercise I need to land the bar on something, so that's why the smith machine is a good machine for that.
Personally I don't notice that much difference though, the smith serves it's purpose for me. If I want to use more glute and ham I simply put my feet a bit further under the bar, so I have to stick my butt out a little bit more. I feel it instantly connected more with the glutes.
I've never had any knee problems. Perhaps other people with more sensitive knees might have problems.
