I have been in a few; though mine are typically ones where both humans are zoo exclusive.
The positive and negatives are largely the same as you encounter in any serious relationship. For the positive we get the social aspects of having another person to talk to, someone to share the economic burden of living with, someone to share household tasks with, someone to plan and work on long-term goals with, wtc; all the same stuff you find with more typical relationships.
The negatives are much the same as well, sometimes jealous issues can crop up, we can be distracted by other events and be inattentive to the other when they need us, failures of economic or hoursehold responsibilities, etc; again, all the same things as other relationships.
The same tools often apply for maintaining and repairing it too, we need to be aware of our shared, as well as our divergent interests and engage one another from time to time in activities, we need to keep open the lines of communication, when we have conflicts we need to try and own our feelings and not use accusitive language ("I feel ..." vs "You make me ..."). Most of all, we need to be ever ready to find any excuse to forgive conflicts and transgressions, truly forgive and not to harbor resentment, and to own up to our mistakes when we're the transgressor.