I... am pretty crushed right now, so I really apologize for my negativity.
However... I have 2002 hours on Civ 5 on Steam by now and have beaten Immortal with every Civ. I have counted the days till BNW for three months now, this was my most anticipated thing all year. And now... wow.
Maybe I was influenced by all the "All AIs are super peaceful" posts here, but for my first, long anticipated BNW game, I chose to play peacefully (which I haven't done... ever, I guess, definitely not on Immortal), also because I was super hyped for all the new additions to Culture and diplomacy.
I immediately became super best friends with everyone on my continent and everything was great... getting my first ever Great Work felt really awesome.
I was badly behind in tech and my friends were building up ridiculous armies, but I truly believed all the posts I had read about them never attacking...
Then, within one turn of each other, two of my Superfriends declared on me. Turns out a lovingly crafted "Oncle Tom's Cabin", gotten through crippling my production in the capital for innummerable turns, is no match for thirty composite bows.
Even if I had done nothing but crank out military units for the entire game (which would have meant no gold, even less science, no culture), I couldn't have matched those armies.
Well, that was six hours of my life wasted. I guess tomorrow it's back to good old non-stop killing everyone, because apparently, that's still the only way for me to win on Immortal.
Again, really sorry for the rant, but... wow, I was so psyched for the game, you wouldn't believe.