I found the story interesting, although it clashes with my personal theory on how AI's and humans will eventually coexist, i.e. human being will eventually become "post-human", by either using genetic manipulation or cybernetic neural plugins to transcend vanilla humans 1.0. I doubt humans 1.0 will be enslaved or hunted down, but they will probably be marginalized (i.e. they just won't matter) as post-humans start running everything outside of Earth. It's hard to say I liked this story, since I thought there was wwwaaaaayyyy too much anthropomorphizing of the main robot character. My computer science training may have gotten in the way of this, but true AI's will neither think or act with anything remotely resembling human intelligence with an inner dialog (heck even a sense of "self"), and it's pretty self-serving to think that the apex of AI development will somehow be a human-like robot.
As for the discussion here about mind manipulation, I personally don't believe in Free Will, as it assumes there's a separate "something-ness" that goes beyond your memories, neurons, and brain chemistry. I take a "but-for-the-grace-of-God" approach, as I think any of us put into the exact same situations as anyone else would do nearly identical things.
I think an illuminating question to ask might be: What if it's not robots giving out the happiness virus? What if it's just humans? If we discovered a completely harmless way to make people be happy, compassionate, and benevolent to each other, instantly solve world problems like war, hunger, pollution, etc, wouldn't you be morally obligated to do it?