The EcoRunner is a "green" product in that the primary material is not neoprene, but "a revolutionary environmentally friendly material." I'm not sure what that is, but I can say I like it better than neoprene -- it feels just as soft, but the cells are smaller and thus the material is more supple. The EcoRunner can be worn on the wrist or arm (using an extender strap, included) and is designed for the 4th-gen iPod nano. There's the requisite vinyl window for controlling playback and viewing the screen, plus the case can be used with the Nike+ Sport kit attached (there's a strap to keep everything in).
Does it work? Well, there's one caveat to our test: we used a 1st-gen nano, the white model that was infamous for being easily scratched. It's increasingly tough to find cases for this model, but we found the EcoRunner is stretchy enough to accommodate the slightly thicker model. The side effect of this, possibly due to thickness or the more squarish design: the vinyl window busted (see gallery), leaving the unit exposed and capable of falling out. Call it a "stress test." It's definitely a design challenge to secure the vinyl in a way that won't get too stressed, especially in cases where everything else is so stretchy.
The softness of the case can't be understated. It is really smooth and lightweight and stretchy. I'm partial to wrist straps, because I tend to skip around my playlists depending on my workout. If you find yourself monkeying with your nano during a workout, this is a terrific wrist strap. The extension strap to use it on your arm adds virtually no weight, and holds up well, but bulging biceps might strain that window. The Velcro used is very high quality, but the stitching on the end can sometimes be a little tricky to thread through the grommet -- so I recommend not removing it entirely. Just pull it off your wrist. One upside is that you can charge the nano in the case, so the entire unit becomes home to your nano. If you only use your nano for working out, you'll never have to remove it from the case.
After a month (OK, nearly 2 months) of solid use -- we're talking about near-daily use on the treadmill and at the gym -- the EcoRunner holds up well. As I said, using a 1st-gen nano isn't recommended by us or the manufacturer, so I don't know if the thinner nanos would bust the window after prolonged use. Other than that, there's very little visible wear-and-tear, and I'd recommend this especially if you a) prefer your nano on your wrist, and b) need a case that'll hold the Nike+ Sport kit. Your choices are pretty limited when it comes to both of those criteria, but the EcoRunner is a good bet either way. If you've had experience with this particular model, let us know in the comments.
UPDATE: Great questions in the comments! As far as sweat retention, since my wrist doesn't sweat too much, I'd have to say no problems there. You might have an issue with a very sweaty bicep, but I feel the material's tighter cells (as opposed the larger, more open cells found in neoprene) would guard pretty well against this. There's no signs of staining, either.
The extender strap adds a little over 6 inches to the overall strap length. Total that's 11.5 inches (see added gallery image). My puny biceps fit fine, but if you have some massive guns then yeah, you might have a problem.