I bought TFIOS as soon as it was available and cried my way through when I was 14, and then raved about it everyone. I read the entire book in one night because I couldn't put it down. I don't think I'd be as captivated by it now, though, and I wouldn't recommend to it an adult who is a serious reader because it is likely to be unsatisfying. It's a "sick lit" young adult novel written by an author who is a young adult. I read all of John Green's earlier books between the ages of 12 and 13, and didn't feel they were too explicit. I loved them all then. I don't know if I would have if I'd read them now. Many books I've read for school have had way more explicit scenes, including ones that are considered to be classic literature.
So, personally, I think it's fine for a Christian, and especially for one who is an adult and not as impressionable anymore.
If you're really concerned, though, you could probably find Christian reviews of it online. One of my friends has incredibly strict parents who read through reviews on a Christian site of every book and movie she is interested in first. She's shown it to me before but I can't remember the exact name. I think it's operated by the Focus on the Family people. They also have reviews of TV shows. It breaks reviews down by a general synopsis and then delves into specifics about what they feel is a positive and a negative for Christians, and giving recommendations.
