@Viakana Well, so far the only ex we've seen is an absolute scum bag, so if we use him as a standard for any other exes she may have had then Chihiro is an angel and will never be abandoned by Nagi. The most potential "abuse" that Chihiro has done to Nagi would be the whole "pet" thing, but that was instigated by Nagi to begin with and Chihiro treats her "pet" real well. This relationship isn't the healthiest, but it's not necessarily unhealthy either and Nagi was honest from the begining about being Chihiro's stalker, so I suppose it's fine. For now, anyway.
Edit: As for Nagi's endgame, we know that Nagi is filthy rich and never has to worry about whether or not she can make ends meet. Thus, one of the things that come to mind is she just wants love, or at least someone who doesn't abuse her and maybe even someone that depends on her in a way that makes her feel good. This could explain what happened with her exes, where they depended on her to some extent and she ended up spoiling them rotten. This time around, though, I suspect she's going to be a bit more careful with Chihiro so she doesn't end up like her exes. Or alternatively, it's also possible that Nagi has been stalking Chihiro for a long time and all of Nagi's exes were just practice so she could figure out how much she could spoil a person before it became too much.
Honestly though, in this current relationship you could argue that Chihiro is more the pet than Nagi is since Nagi takes care of Chihiro, but no one said they couldn't be each other's pet.