Not to toot my own horn (toot-toot), but several reviewers have commented about my "Singularity" being an especially positive story, not because it ends on a happy note for the protagonist, but because it reflects on a happier outcome for humanity in general. However, on the same note, I've gotten responses and reviews from many who find the entire premise (of a positive human response) to be "unrealistic and hard to accept", so you'll have to decide for yourself. If you're skeptical about how good people can be, skip it. If you believe in the power of humans to reach solutions, then dive in. It's NOT a do-over, however.
In general, though, my stories tend to be a bit darker than most. I tend to kill off a lot of characters, often the main characters too, as I like the idea of self-sacrifice to reach a common goal.