What is the coldest temperature it can be used in?

I've used this in temps down to freezing with good results. The colder the temp, the longer the cure time. But this product will cut that time - whatever it is - significantly over the regular concrete mix. I don't know from experience about temps below freezing, but from what I understand about the chemical reaction involved it should work just fine - the temp would just affect the cure time.

The coldest temperature will depend upon the ability of the project site to be conditioned so that the QUIKRETE Fast-Setting Concrete Mix does not freeze during the first 48 hours after being placed. This is critical. It is important in the cold weather to take certain precautions: 1) Mix the concrete with water before placing the material (even if it is being used for a fence post), 2) store the bagged concrete in a warm temperature before use, and use warm water to mix, 3) protect the concrete from freezing for at least 48 hours (for a fast setting concrete) using plastic sheeting, insulated blankets, or other means. If these preventive measures cannot be taken, it would be best to wait for warmer weather.

Doesn't matter. Cold will help it set faster. Just make sure your water is not ice when your pour it in...

Above freezing. Above 40 is better, but concrete produces heat when setting up, so it shouldn't freeze at near-freezing temperatures. It will NOT set up and cure properly if it freezes in the cure period of time, though, so if you HAVE to pour concrete and it is cold out, cover it with plastic sheeting and then a blanket.