What I like about this movie is that it doesn't give clear answers what's good and what's not. This is how war stories should always be portrayed.
On one hand I was rooting for colonel Nicholson, because he was right in most cases, they really turned defeat into victory by building the bridge, every soldier working on it felt like soldiers and not slaves, it proved to japanese how strong british nation is etc. (And man oh man, isn't Guinness a fantastic actor?)
I was also rooting for colonel Saito. Even though he was a jerk at the beginning, he had his reasons. His life was depending on this bridge and he wasn't expecting the british to co-operate, so he was cruel to them. One of my favorite scenes in the film was when Nicholson convinced some soldiers that needed medical attention to work on the bridge and Saito was looking through the window with his mouth open. He was probably thinking of letting Nicholson go back home after the bridge is done. Saito changed a lot throughout the movie and I was not expecting that, usually characters like that stay evil all the way untill they are killed.
American and the rest of his group were right as well, but I was rooting for them the least as I was hoping the bridge to stand.
And the finale was uncomfortable for me to watch and that's fantastic actually. Again this is what war stories should be doing to you. You should never know who to root for because in war there are is no right side.
The most disappointing thing about the movie is that it's not a true story. It's based on some true events, but my favorite aspects of the story didn't actually happen.

At first the movie seemed pretty ordinary, but it just continued to build. And then that ending... wow.

Looking at this movie as a whole, it works brilliantly in conveying its theme of the madness of war, and how it drives good men to do bad things.
As I was watching the movie, for the first hour I felt increasingly annoyed by Col. Nicholson's "play by the rules and do what you're told" philosophy. Then as the movie went on, and we follow the British plan to blow up the bridge, I realised that this was how the movie wanted me to feel, and then the finale pulled off a brilliant reversal, and a sort of redemption for Nicholson.
Standout scenes are the shooting of the Japanese soliders with the clouds of bats whirling over the forest, and, of course, the incredibly tense and powerful finale.