I just watched it. I was very impressed by how they pulled off the battle.
I always thought the scale would be very small for the show because of the cost. I think the changes they made were necessary for time constraints. The green fire looked great, and the explosions with splintering ships looked good. I loved the long shot of the fire arrow making it's arch into the water. Moreover, I think the sense of chaos with the pyromancer fire, nervousness of the women waiting in the room where Cersei was, loss of moral at the mud gate, and shock of Tywin showing up were conveyed very well.
The battle in the book was just unbelievably epic with Joffery having the naked bodies catapulted into the blackwater, green fire all over the ships and the surface of the water, drowning men, ship ramming, and the Hound and his men fighting on the shoreline and boarding ships was amazing to read.
My only nitpick was how they messed up the single greatest line in the book when The Hound was offered water and he just said "Fuck your water!" I've gotten to urge to say that at restaurants when I've been offered water ever since. He just mumbled "fuck the water" in the show.