Now I use trekking poles for just about any hike imaginable. In my view they ease the strain on knees and ankles (my right ankle in particular is a problem), improve your balance and make it easier to negotiate tricky terrain, let your arms share some of the load with your legs, and even help improve your breathing and pacing.
As for which poles, it's the poles themselves, regardless of style or fanciness, that are the big improvement. After that it's hair-splitting. So a pair of $55 REI Summit poles (www.rei.com)—very basic, three-section poles—are basically the equal of a pair of Leki Super Makalu Cor-Tecs, which have ergonomic cork handles and a built-in shock absorber and sell for $140 (www.leki.com). Yes, the Lekis are nicer cut of pole, plus are slightly more comfortable to use. But the big leap is from no poles to poles.
A good middle-ground set of poles would be the Black Diamond Contour Trekking Poles ($100, www.bdel.com). They're light, sturdy, easy to adjust, comfortable to handle, but don't have the added complexity of the anti-shock feature.
I will say I've gotten reports of trails with lots of rocks and boulders where pole-pokes have actually scarred and defaced the rocks. I haven't seen this myself on hikes such as Washington's Granite Mountain, where the final stretch to the summit is a boulder-hop across lovely granite stones. But, keep it in mind.
Otherwise, pole on, my friend. You'll love it.
Poles, boots, and other trail treats reviewed in Outside's 2004 Buyer's Guide .