What nuance ‘I’ll leave it at that' carry? Does it mean total agreement with, or commitment to your partner?

I'll leave it at that simply means, "I could say more, but I've said enough – there's no need for further discussion."

I'll leave it at that could indicate strong agreement, or serious reservations. In the former case, it might substitute for something along the lines of "I don't want to keep on gushing, and heaping the praise, lest I start sounding ridiculous." In the latter case, it might mean, "I don't want this to turn into a nasty argument; I've said my viewpoint, and there's no point in escalating this."

In this case (in the wake of the recent controversial Akin remarks), candidate Ryan doesn't want to venture too far into what's become a political minefield, so, he summarizes Romney's position – tersely – and then declares that he won't be saying anything further on the matter.

Other similar ways to paraphrase this sentiment, or something close to it, would include, and that's that, or, end of story, or, I've got nothing more to say.

I could elaborate more, but I'll just leave it at that.

Leave it at that: to agree that there has been enough discussion, study, etc. and that it is time to stop

Let's leave it at that for today and meet again tomorrow.

Leave it at that: (informal) to take a matter no further

Collins said: `I just want to announce my retirement from the Scotland team and leave it at that.

Don Scott is highly respected for what he has done on the field and let's leave it at that.