[Nyororin]

And... Let`s not turn this into some sort of flame war.

My understanding of the meaning of ダンディ is that it is high-fashion masculinity.
A man with a well trimmed mustache, who is muscular, but who is very fashion conscious would be considered ダンディ in Japan.
I do not believe that the meaning has a really direct link to the past meaning in English.

I will go completely against what KyleGoetz is saying - ダンディ is a lot closer to GQ than "metrosexual". Think fashionable without any femininity. (To me, metrosexual means some embracing of feminine features for the sake of fashion.)
Looking at some random GQ covers on GIS and finding that there are some that are the very epitome of what I would consider ダンディ.