I could accept the remastering with new scenes in TOS - 60s budget wasn't exactly something that lent itself to a sci-fi series, and part of the reason my TOS tapes never saw as much use as the recorded TNG, DS9, and VOY ones I'd made was because by modern standards, they looked strange - 'evasive manuevers' involving the ship moving lazily to the right and stuff like that, and the phaser effects seemed silly looking. If I watched them now, I might actually consider it nostalgic, but I think the remastered has a better look, even if there's a clear difference in quality between the stuff on the viewer and the stuff on the bridge.
However, I think that the movies and the later-era Trek (re: TNG, DS9, VOY, and ENT) should be left alone, because they look perfectly fine. Okay, the odd touch-up so the phasers aren't firing from the photon torpedeo tubes or something wouldn't be unappreciated, but for the most part, leave it alone. Nowadays, there's such a reliance on CGI, it's nice to go back and see the good old fashioned physical models. There's something I feel is more real about the Enterprise-D and DS9 because they were so rarely rendered in a CGI model. I miss that in modern shows. Sure, updating the effects to look 'more real' will make everything shinier, but there's something about them that feels less real to me. Maybe it's little more than the difference between replicated and physically prepare food or something like that, but I want my physical models and older effects left alone.