So how useful it is in practice?

It's useful for quite a few things in practise:

• You can be sure software that is packaged with nix will behave the same on different computers.
• You can avoid dependency conflicts.
• You can automate some things that would otherwise take multiple (mostly manual steps) on other systems.

This video shows off some of the cool things you can do with nix: https://youtube.com/watch?v=6Le0IbPRzOE&feature=share9

How do updates work?

You update a programming by specifying the latest version of a program in config and rebuilding.

You update the OS by pointing to the channel you want to use and rebuilding.

You can time travel back to a previous state if anything goes wrong.

Can it play Crysis?

I expect so, some people.do use nix for gaming.