The original Star Trek is a show I've always liked, and if anything it's grown in my estimation over the years. I was born in 1987 and didn't see any Star Trek at all until the second decade of my life, so nostalgia doesn't enter into it. TOS wasn't even the first Trek I saw - that was Voyager, a show I now easily rate as my least favorite in the franchise.
Though the original is clearly a product of its time (and how could it not be?) I don't think age has done anything to diminish what makes it entertaining - the most downright entertaining of all the shows, in my opinion.