Recently used this plugin for something far outside of WP's comfort zone and it handled it like a champ. In fact, I couldn't have done it without it.
The dev API gave me all of the flexibility that I needed to get the job done and the documentation was pretty fantastic.
The interface is solid and not something I would be ashamed to put in front of a client as is so often the case. Really smooth.
And I barely scratched the surface of what you can do with these relationships. I merely linked posts together. But you can add so much information to the relationships complete with beautiful interface support.
Give it a try.