Funny you say that, I only every use premixed, partly because its convenient, I don't have to find another gallon jug, and I also keep an unopened bottle in the car at all times for emergencies. Opened bottles tend to leak. Never had a low coolant light problem.
I know its a worse value, but I prefer to work quickly and get things done so that I can do something else, rather than spending time mixing something I can just get premixed. I do recommend the value brands, its all the same stuff and will work fine so long as you replace it often enough.