Virtually all the Gender Bender mangas I've read so far have had the following plotlines:
guy/girl is changed to girl/guy
guy/girl crossdresses due to overbearing parent/guardians
guy/girl crossdresses to get closer to someone
guy/girl gets transported to the past and is viewed as the opposite gender
guy/girl has an identity crisis
Nononono, on the other hand, breaks the mold and has a realistic reason to which you can relate to:
girl pretends to be guy in order to fulfill her olympic dream which only men can participate in
Now, before all the naysayers point out that men are physically superior to women, the limits of some women do sometimes come quite close to the limits of men. Additionally, the olympic event she is aiming for is a team event, meaning achieving her dream is a real possibility, assuming she has the potential and skill (which she does).
However, the reason for crossdressing isn't everything to how good a manga is and thankfully, Nononono delivers on this point. The plot is furthered at an ideal pace - neither too slow, nor too fast, and while some of the character's personalities are a bit unrealistic, it does not detract from the manga at all, instead meshing with its somewhat dark plot and making it all the more better.
All in all, this is one manga I definitely recommend. Forget any bad experiences you may have had with the Gender Bender genre because this is one of the very few shining examples of it done well.