I finished it the day it was released. Just did not understand one concept though. How on earth Dumbledore defeated Gellert when the later had elder wand?
The only theory that I could form was that the elder wand does not support the wizard unless he truly overpowers the previous owner. That is he can still do good bit of magic but not extraordinary magic.
Gellert ownly stole the wand from Gregowitch. So, the wand granted him partial ownership and did not let him become invincible. However, when Dumbledore defeated Gellert, it truly became his. So, even Voldemort could not beat dumbledore. He had to flee.
Well, I am sure this is discussed all over the web and some experts have much better answers. Still, I do not have time at this point to study this more.