In the comments on this post at Amy Welborn's.
The whole discussion is excellent. Comments from people whose experience was great, from those whose experience was not so great and so they quit, from those whose experience was not so great but they didn't quit, from priests and seminarians, from people with medical difficulties, from NFP instructors.
We've experienced NFP mostly positively, with a few very trying times during my brief periods of lactational amenorrhea. All our pregnancies were planned, but then, we tend to practice NFP pretty conservatively, and Mark tends to be more conservative than I do about defining the edges of the fertile time (which, anecdotally from talking to people, seems to be less stressful on a marriage than when it's the wife who wants to be more conservative).
One thing I can say unreservedly is that, although it has at times been very challenging, I don't think we've ever been through a time when contraception has looked like a better option. I can think of a few times when TOTAL ABSTINENCE has looked like a better option.