like doug, once i tried them, i knew i'd never go back.
why use them? easier on the knees downhill, help pull yourself uphill, safety on snow and stream crossings or any loose ground, more protection against missteps and sprained ankles, and hold up the front of my tarptent. i'd never do a hike without them. i hate climbing hills without poles! and even on flat ground, they just help me get into a good walking rhythm.
recommendations? i've always used adjustable poles - all from komperdell. the internal locking mechanisms sometimes get a little finnicky, but you learn how to deal with it if it messes up. i had one break inside, but that was after lots and lots of trail miles, and rei replaced them right away (trekking poles are one of the few items i always make sure to buy at REI!). they make two and three section poles - i always have three sections, as they collapse more for travel or stowing on your pack. the two section ones would be fine i suppose and would have less to go wrong, i suppose.
i just got carbon fiber ones, and am very happy with the weight and stiffness. others will disagree, but i removed the wriststraps off my poles. i find that i don't need them, and they tend to just get in the way when reaching for the camera or a water bottle. i never thought i'd take the straps off, but after a few weeks on the pct, i just found myself never using them, so i cut them off. never looked back. but don't get rid of them until you try it out a lot with them on! the only real time i would want them is on a sketchy snow slope or sometime, where if i dropped it, i might not get it back easily... but if it's that steep, i usually would have an ice axe out, not poles.
i can see the appeal of a fixed length pole, once you decide the length that works for you. it's usually somewhere around where your arm is at a 90 degree angle when holding them. perhaps longer going downhill, or whatever. i tend to put them where i like them and leave them. the fixed length pole might not be a good fit for a shelter though, unless it's the right height, or you angle the pole (a la tarptent contrail).
ignore the (now dying?) hype about shock absorbers, imho. an unnecessary gadget.
viva trekking poles!