Here's what I did...
Having fixed four pairs of jeans over the weekend (not all of them mine!) I got this down to a fine art. I wore these patched jeans this week I can say that after the first few wears, I didn't notice the patch anymore. When I bend over, there isn't as much give in the back of them as there was - but that's probably a good thing (and reminds me to bend my knees more).
Its a good quick solution to keep wearing jeans that otherwise are good. This works with knee patches as well as long as its a tear and not a gaping hole. We sacrificed the oldest most worn jeans we had between us to patch all the other jeans but any solid, thick robust material would work - but its going to show through the patch when its finished so pick a colour that you don't mind people mistaking for your undies!
Green-ness: 5/5 for not sending something to land fill
Frugal-ness: 5/5 for using things you already have and not spending any money at all.
Time cost: About 20 minutes a patch - some of its a bit fiddly.
Skill level: Back and forth straight line sewing.
Fun-ness: It was fun to present various family members with their favourite jeans able to be worn again!