I've bought one or two vintage bottles of Kouros and Aramis 900 and found them to be fine. I certainly wouldn't hunt them down and pay any more than a new bottle though.
Beware posts on Basenotes raving about the superb quality of the older scents and bemoaning reformulations.
There's a reason the old stuff smells a little bit different to the new stuff. The old stuff is just older, it's sat in a bottle for 20 years. Take a new bottle and leave it for 20 years and it'll smell differently to something you'll buy in 2038. Not better or worse, just very slightly different.
Fair dues, oakmoss has been removed from a lot of scents due to it causing AIDS or global warming and scent-geeks will tell you that has caused everything to smell different but I'm pretty sure the perfume wallahs at these firms managed to replicate the smell of oakmoss.