I liked Bokurano but yeah it's a very dark story. There are some similarities to the darker aspects of Puella Magi Madoka Magica for example, children forced to fight to save the universe with a hopeless personal outcome as their ultimate reward. There are at least a dozen children ( I forget the actual number since it's been a long time since I read it ) and they all have different backgrounds and personal stories. A lot of the manga is looking at the children's lives and how they deal with what has happened to them. I think it is worth reading but it definitely won't leave you with happy feelings. In fact I read somewhere that the director of the anime thought it was too bleak and got permission from Mohiro Kitoh to change things somewhat so the anime isn't as dark as the manga.

@lucamarte - You really should watch the Yokohama Kadashi Kikou OVAs, especially the first 2. If you remember in the manga when Alfa goes to the hill overlooking the seashore as the lights of the old city now under water come on and illuminate the outline of the former shoreline. Well they animated that so beautifully that my jaw almost literally hit the floor. I started playing the lottery now even though I think its stupid, just so that if I win millions I can head over to Japan and pay them to animate more of the manga. :)