Gold Rush has been unexectedly successful for me in central MN, zone 4a.
Hardiness is not absolute with one tree dead but the other one
surviving -30F two winters of its 6 total years planted. It never really
ripens on the tree - when it has to be picked in November for cold it is
still very green and hard as a rock, but ripens to great flavor in storage.
I've got about 100 seedlings from crosses with 4 earlier and hardier apples
including Zestar and Goodland but won't see any fruit from them for a couple
of more years. Gold Rush is a very clean apple too. So you northerners don't
shy away from trying this one because it's not rated hardy and you can't eat
it off the tree, it's differences from other apples extend to the great
flavor, storage time and other fruit quality too.