Plastic (polymer) holsters v. leather. Leather looks better and is classier. Plastic can be thinner, stronger, and more durable. I think nylon is a good compromise.
Personally, I have no problem with them for plastic (polymer) guns. However, for a revolver, 1911, BHP or other classic pistol they are an abomination. For steel/alloy but utilitarian guns like the SIGS or Rugers I'd still prefer leather (or nylon would be OK) but polymer may be OK.