OK... I'll start with the realities.
I've held off reviewing these until I was sure about things, so I don't do a inaccurate review based on "out of the box" conditions.
I have the Micromodal (my god that fabric is soft! And stays soft on washing!) and the Fast-dry long-legged boxers (not as soft, but not as "nylon" as you might think either).
Point 1: They take some getting used to. Things aren't where you are used to them being. But they really do feel like you have nothing on, once you get them on right.
Point 2. The fabric is really soft and light: To the point I was worried about the washing machine eating them. So I have invested in a couple of net washing bags to wash them in, (NOTE TO SEPARATEC: Maybe you should sell them or include them in the box with the pants...)
So far, no issues. I think the longevity should be fine (so long as they don't snag in washing), I have had no loss of shape or elasticity, and they come out of washing (even without fabric softener) just as soft as they came out of the box.
I have been wearing them (and so washing them) quite a lot, and indeed have bought 3 more pairs (normal boxers). I will say, I am a convert to long-legged boxers though.