From the link above I picked up a Midnight Blue and the "Fog" colour. They arrived today, so it's definitely something I wouldn't pick up if I wanted something new in a hurry, but for a tenth of the cost of a real thing I'm super happy.
Very minor differences between the two. I can't tell if the material is different than the original because my black band now has a slightly worn feel to it, but it certainly feels very similar. Second photo below shows that the real thing has the size stamp, while these particular non-genuines don't. Clasp "action" is also slightly harsher on the non-genuines, due to a slightly less rounded pin/thing that inserts into the band, but otherwise they're identical.
Still awaiting my 42mm modern buckle leather band, which Apple don't actually offer. That one should be interesting, but it was about US $28.