I think the same as most of you, the art is the first impression, wether it's the cover so I click on it, or the first pages I read, but if the story doesn't follow, I would stop reading it.
I think so-so art is be okay as well, but if it's bad enough to be distracting, I might not be able to read it at all. Though on that point, I think everyone's criteria are different.
For exemple, I don't think Mob Psycho 100's art is distractingly bad, even though I definitely won't call it "pretty". It was enough so I start reading, and the story was amazing so I continued. On the contrary, I've started amazingly pretty comics, but stopped reading them because I got bored.
Conclusion, I think the art must be at least enough to catch the eye, but that the story is more important if you want to keep your readers.