Well written. For it's genre.
What genre? Romance? No it isn't. It's awful. Gothic fantasy? No it isn't. It's awful. I admit, romance is not known for its literary merit, and any type of fantasy is just above **** and Jane, but Twilight still manages to be bad. If you mean it's well written for its genre of "crap" then I'd say the quality of writing is irrelevant. It's what... well written for being as well written as it is?
And I'm not sure what you mean when you say it's a good franchise. It makes money and is popular? Sure. But slavery made money and was popular too. Doesn't make it good.
All in all, while I think Twilight also sends very bad social messages, for the most part it just is what it is. Horrible. And if anyone likes it, that's okay. So long as they don't go thinking it's good.