usually the magnification is when used with a 50mm lens, and when focused to infinity. as long as this is constant, you can compare between cameras which gives a larger view. i assume bigger is better! the canon 20d (0.9x) and 5d (0.7x but full frame!), and the dynax 7d, are some good ones. the nikon d70 (0.75x) gives a tiny view.
but u must also keep in mind the viewfinder *coverage*. this means how much you can see of the actual scene that will be recorded. while we would expect 100% coverage, only pro models offer that. the rest are 90+%. usually if you get 100%, the magnification is a bit lower, but the viewfinder is still large. e.g. the nikon d2x, 100% coverage at 0.86x.
the figures above have nothing to do with brightness. manufacturers don't state that.