I guess the only one who can answer that question is the slave her-/himself. And I doubt that a slave would be 'in it' if they didn't feel happy/appreciated/loved, at least once in a while.
But then again, I'm merely a submissive kinkster, so I might be completely wrong.

As far as rights go: A slave has exactly the same rights as a Master, at least outside of the relationship and at least in most parts of the western world.