Just installed a Zone SS about a month and a half ago. No complaints - install was easy, drives nice, great customer service from Allen at AOR. Costs a little more due to the shipping, but it's a much beefier part than stock.
Most are pretty much the same as DevilDogDoc says, a hydraulic shock that is valved 50:50 so it resists equally in both directions. The difference in some SS kits is that not all include the tapered stud required to secure the SS to the drag link. The old tapered stud is a permanent part of the factory SS so you need a new replacement SS with a new SS. Just make sure it is in any kit that you buy.
__________________ When you have a choice, buy American.