I remember seeing a bit of TOS before any of the new series when my Dad would watch it, and I used to like it. I eventually got into Star Trek as a franchise around when TNG ended in 96 in the UK. TNG started up again from season one, VOY was just starting up, and DS9 and TOS were also being shown. I watched everything back then when I was around 12.
These days, approaching 30, I'm a bit weird with TOS. I'll quite happily sit and watch the good stuff over and over again, but the bad stuff really turns me off. Of the DVDs I have, season one has had the most use, season two has had some, whereas season three hardly had any. Probably not that surprising.
Anyway, perhaps it's nostalgia as I grew up with TNG/DS9/VOY closer to the time they actually aired, but I can sit and watch bad episodes of those series, whereas not with TOS.