It's very obvious that some batteries last longer in some devices then others and vise versa. It really depends on the practical capacitance of the load put on the battery.
In a contraption
like this you can adjust the load you put on the battery or batteries. But it's a capacitance load. So if you mix the brands of batteries, you'll find that some put out a higher voltage than others under the same load.
In the picture, you can see there's some Duracells and a couple generic battery. The Duracells put out 1.5879V while the generic only put out 1.5544V. The end result is that with a 149.7mA load, the Duracell lasted 338.8 minutes, while the generic lasted 384.1! So in a flashlight, the generics actually lasted longer. But in something like an MP3 player where the load is lower, the Duracell may last longer.
Pretty neat.