I sided with the NCR, but I wiped out the Khans early in the game. I hadn't intended to. I forgot about the faction armour triggering hostility and it was self-defence, I assure you.
I was quite excited when I finished the game and immediately started a second, so I could explore other paths but quickly got bored. Maxing out at level 30 long before you get near the end of the game is a bit of a drag.