How Does Ranked Matchmaking Work?

Does anyone know the details on how ranked matchmaking works? How am I matched with teammates/opponents? Does my win or loss streak influence who is on my team (better or worse player)? Are statistics from games taken into account for any reason at all? Or is everything just MMR based, random players assigned at that level? Are promo matches matched differently? Is matchmaking manipulated to try to get you to have a 50% win rate at each tier?

Anyone from Riot care to comments?

Personally I would prefer that if I am in S5 that I be randomly teamed up with other S5 players. Once you are in a tier, your MMR really shouldn't matter that much for matchmaking, just for tracking your movement up or down within a tier. If you had a high win rate, you would move up to the next tier, working toward a 50% win rate at your skill cap, at a tier playing with equals. I am curious as to what is actually happening though... help me understand Riot!