I used to think they were useless and at times downright dangerous to people around them.
But after seing a couple of people who worked for Outward Bound using them and then trying them, the only time I don't use them is if I forget to pack them!
Some peple say you don't need them on flat terrain and they are a PIA, well stow them on you pack if that's the case.
I do anything from off track steep climbs out of gorges with 500m ascents (or decents) over 2-3km carrying a 20kg pack to 30km walks on realtively flat terrain.
I'm a fit person in my late 50s with good knees (but dodgy ankles) who still jogs 4km twice or three times a week.
Price, start with some cheap Coleman type brand versions, you can pay $300 if you want the latest NASA weight saving materials. I've got a $70 pair of Black diamonds, I've tried the shock absorbing Leki's and found they didn't do much for me
Borrow some and see for yourself