I was never really a fan of the original series. I like the characters but I have a hard time getting into a show that is so old.
I have seen every episode of The Next Generation, and most of Voyager and Enterprise. I just started watching DS9 for the first time.
in twenty more years, TNG will be just as "old" as TOS now seems. Old is relative. You should give TOS another chance.
DS9 is a great show, you're lucky to be just starting with it.