My '71 D-18 is gorgeous ands sounds tremendous. It is the perfect blend for folk, country, and bluegrass. The farther you get into the '70s, the more chance you will get one that might not be as good. However, they can't be all that bad either. I have followed the vintage guitar market for quite a few years now. Even those mid to late '70s D models go for some pretty good bucks and are still in demand.

Call it a mystique or call it what legends are made of. Martins are the standard of the industry and the brand name sells itself. Like bobert says, everything else, especially Taylors are wannabees and their resale value compared to a Martin speaks for itself.