My wife gave me a set of trekking poles for Christmas. Going down hill, they are the bomb. They take a lot of strain off of the knees. They also are good for general stabilization, especially with a heavy pack. The difference between the cheap and more expensive ones would be both weight and durability. The carbon fiber will have more flex and less weight than aluminum. I'm sure there are differing qualities of carbon fiber. For $19 you can't go wrong for trying them out. I sunk mine into some soft earth as I slipped on a wet hillside this spring. A cheaper pole would not have bent that far without damage. Mine sprung back. The better poles have an anti-shock feature that allows some spring through the length of the pole. Cork handles allow for dissipation of sweat. The adjustable length shines both for finding the general fit for you and also are nice for adjustment to shorten going up hill, lengthen going down hill, and one of each when side hilling.
I believe she picked mine up from REI, on sale, for about $70 for the pair. They are one of those things that I often thought would just be extra weight and bulk, but are a real asset in steep terrain.