I also have knee problems like many of you, but I would probably use poles even if my knees were perfectly healthy. Using poles as supports for creaky joints is important to me, but I also use poles on all trails (flat, uphill, downhill) to increase my speed and endurance. I use a "nordic" style to push off from behind to propel myself forward.
Doug Johnson explains it best:
"First, in my opinion and experience, using trekking poles can be easier on the body and also more efficient over long mileage. I started out a naysayer but converted after using my wife's poles on some long days and through deep Utah sand. Now I use poles for every hike.
Second, there are two main styles of pole usage:
"Trekking style" mans you use the poles for stability and balance. The tips are place beside the feet as you walk and they take weight off the knees and offer additional security. These poles tend to be shorter typically fit the 90 degree rule mentioned above.
"Nordic Walking style" means that the poles are used for forward propulsion, much like a XC ski pole. Sized for this style, the poles are usually a bit longer (mine are about 5 cm longer). Here, the poles are place behind the feet and you push off for increased speed or forward momentum.
That said, you'll likely develop a blend of the styles, although you'll typically fall into one camp. For example, my wife uses trekking style 90% of the time but uses NW style when climbing steep sections. I use NW style primarily but switch to trekking for stability through really rough sections or when descending sketchy or very steep parts."