I agree with Winning Guy when he says it sounds too good to be true... but, at the same time... maybe that's a benefit of being locked-in to 24 hour check-ins and the process to sell your games?
The way I've been assuming it will work, because they keep stressing "family members can access your shared library", is that you just remove a current game you are playing from your "shared" list so no other family member can play it and yu wouldn't be locked out from playing. OR, They could also put in a system where if the content owner (me!) wants to play the game, it does an insta-save for the family members currently playing it, and gives them a courtesy message letting them know that the content owner has priority access to that game... something like that. That type of scenario allows for you to "always" have access. OR, maybe you can always play, and one other family member can also play at the same time, but you just can't be in the same games together (i.e. same multiplayer lobby, or whatever).
This is an amazing feature. My brother is EXTREMELY limited in his scope of games, but when he was over for Christmas and saw me playing Skyrim, he was hooked, bought it, and now we talk about it all the time. Point is: I can now showcase games for him without any upfront cost for him at all... and I assume if he likes it enough, he will purchase his own copy so we can game together. I know I want to try out the new FIFA 14, but I'm not interested enough to purchase it without playing - as my brother is. He will share the game with me so I can try it out.
This is the power of the Xbox One.