Bokurano is a pretty original story. If you find yourself connecting with the characters, it will get really sad for you really quick.
DO NOT READ PAST THIS IF YOU DON'T WANT A MAJOR PLOT POINT SPOILED!
The Zearth, the giant mech, is powered by the lifeforce of the pilot. Ergo, once they finish a fight, they lose their lifeforce. This means that in order to save the world, they have to sacrifice themselves.
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I enjoy the story, mainly because it is executed very well. The mecha action isn't the driving force, so don't expect much in that area.