The extensions work well enough, though there is some added risk of losing air through the "extra" connection or a cut in the extensions tube. Personally I would choose the ones with a braided metal cover, but the rubber clad ones are pretty sturdy too. You also need a strudy mount for the outer end of the extension, since it cannot be allowed to "flap around" as the wheel spins. I don't see any mounting brackets on the ebay item, but those sold in auto parts stores generally do have them. At a higher price, of course.
I've used the braided stainless clad extensions on a couple of rigs and perhaps 70,000 miles and never had an extension failure. However,I have had an air loss related to an extension because the extension came loose at the outboard end due to a broken mounting bracket on a front wheel. I replaced the flimsy spring mount bracket with one I riveted to the wheel liner and now have no worries. My current rig came from the factory with the extensions on the duals and those have a sturdy bracket that is held on by one of the lug nuts.
You can get long, angled metal valve stems installed on the duals at many truck tires shops and that is clearly the best solution. However, the extensions do make it practical to check/fill tires regularly and in my opinion having proper air pressure at all times far outweighs any increased risk due to the use of an extension.
