It really depends on how you define watchable. I do find most of TOS watchable and entertaining, just not in the way it intends to be. To me it's pure comedy, just as I enjoy MacGyver and CSI on the same level.
The percentage of episodes I'd consider actually good though, extremely low. A lot of them are redundantly formulaic anyway, and not necessary watching.
I certainly feel similarly. Plenty of those episodes aren't great, but since I'm a fan of Kirk, Spock and even McCoy, I'm always down for watching them do their thing. Occasionally it's even just for one scene, but an episode that leads up to a big payoff is fine for me.
If TNG had even one character as good as them, I'd be cooler with it. The fact is that I just hate all of their boring, lifeless asses, so there's no interest in watching them in crappy stories. Most of what I did watch were Data-centric episodes, or ones with Borg, Q, Klingons or Romulans in them. Anybody but those smug, awful federation twits.
The problem with ENT is that it brought nothing new to the table at all until S3. Up until then, it was just stale and poorly recycled TNG/DS9/Voy plots, and without the strong cast to carry it. S3 was the only great season, but even that had some blatantly recycled plots. Most people seem to love S4, but I thought that was the worst. Unashamed fanwank, most of which was forcing the square peg into the round hole. That season demonstrated perfectly that Trek had painted themselves completely into a corner by that point.
Ironically, the best episodes of S4 were the Mirror universe where they were on the Defiant, because they almost showed some character. Incidentally, it also technically didn't include a single character or moment in the prime universe! Possibly the only example of that in all of Trek?
I remember thinking at the time they should have just said fuckit and made a 5th season set entirely in the mirror universe. Unfortunately, even the mirror versions of the characters were pretty bad. While it was better than most of that season (and the opening credits were probably the single best thing the series ever did), it was really only redeemable for staring the hot, slutty Hoshi.
And for that matter, the one with the augments wasn't bad, although they should have knocked off the entire Sung aspect of it. Either do a pre-Data episode, or a pre-Khan episode. They overdid it with the fanwank there.