As I watched I Am Legend, it became pretty obvious to me that the zompires were smarter than Will Smith's character was giving them credit for. They copied his traps, kept dogs for pets just like he did, and were able to understand his motivations well enough to move his mannequin "friends" around. I believe my summary of the second half of the movie was: "he kidnapped the king's daughter."
The alt ending shows Will Smith realizing that the zompires are intelligent, returning the female he kidnapped, and finally driving away from NYC with the cure in hand, accompanied by the woman and child he met. I'm not sure how I feel about this ending. On the one hand, it does seem more interesting to me because it shows that Will Smith's character can grow and change. He has the ability to get over the traumatic emotional damage he experienced when the outbreak occurred and his wife and kid were killed. On the other hand, that ending does seem a bit trite and Hollywood; ye old "love conquers all" cliche. In some ways, the "suicidal maniac" ending is truer to what Will Smith's character really is.
A hybrid of the two endings might have been most interesting of all. Smith's character can realize that the zompires are intelligent, and return the one he kidnapped, but then either the realization drives him insane and he pulls the pin on the grenade, or the zompires destroy him, or both. The woman and child get away with the cure, but maybe with some reservations about using it on a wide scale.