I've completed Red, Crystal, and almost completed Sapphire. I like playing them, but rarely do I complete them (usually restart after the 4th badge)
The ones I made the least progress in were FR/LG (didn't even get a badge) and B/W (never got the second badge)
Oh yeah, it's not that I don't play the new ones. I've played Red, Blue, Yellow, Crystal, Leaf Green, Ruby, Diamond, and Black (and Pokemon Pinball

)... but not since Red have I ever gotten to the Elite Four.
It doesn't seem like Nintendo really adds much to each game, besides a new roster of increasingly weird Pokemon.