@oliver I mean.. the way you put it, the data mapper is only convenience probably. I think valinor supports stuff like nested levels that ocramius/GeneratedHydrator doesn't for example. Not sure 100% for either.
Personally, I would not consider writing my own data mapper, just for the sake of not losing time with the project.
Recently, I favour the notion of "first version is probably throw-away code" and just try to make it work. It all depends on what level you design things, I guess..