I have an 9 years old HD-28 and some of my friends use other Martin models bought 3-8 years ago. Some of us are professionals and some (like me) are just amateur folkies.
We all share the opinion that Martins before 1968 were much better, esp. when thinking of materials. Our Mguitars need corrections aprox. 2 times a year, including corrections of the neck.
I think Martins dated 1968 and later aren't worth money they cost. Besides we have many other instruments, that we prefere: various Takamine models, some old Guilds, newer Ovation.
I think, this is like other famous brands ex. Mercedes: 20 % values + 80% marketing.