No. I think the reason it is so popular is because Bella, the main character, really has no defined qualities, so when people (specifically girls or gay males) read the book, they can easily imagine themselves as her. They are entertained when they read the book imagining themselves as her, and they like the feeling of everything that happens to her when they read the book, and they recommend it to others -- this is why it is so popular. Stephen King was quoted in an interview to say that Stephanie Meyer (the Twilight saga's author) is not a good writer, but became successful because of how she wrote.

As for the second question, I would take the chance to be immortal if it came around, but I think I would do it only if many others or everyone had the same chance as well. It is the aim of most religions to establish immortality. I would think it'd be a bummer to watch everyone you meet die, though. Not to mention that if we were immortal, we'd have to constantly be moving to different planets, because when the stars explode in each solar system, we need a new place to stay (to avoid floating in space forever). Also it'd be really weird if the universe came to a singularity again (like some scientists predict it will).

I believe people are interested in twilight is because of the amount of interest. People are psychologically attracted to things they and most people are not capable of doing. For example transforming into a wolf. Heck, I would if i could but we can't. That is the foundation of what builds fan clubs and cosplay. People will dress up as wolfs, or growl at people, sometimes it gets really weird.

Also, people are attracted to the suspense in movies. Suspense builds up after a while, people are eager to know what happens next because of their huge amount of curiousity. In addition to people wanting to be immortal or transfer into wolves, i believe that anything can be very attractive to people because of the quality of the writing.