Overall an engaging film (certainly much better than the previous alien invasion movie I watched, "Skyline"). But not a rewatch film by any means.
The action and pace is what makes this story: you never spend too long in any one place. As many reviews have said, this is a war movie, with aliens. By removing a few scenes, and cutting in different opponents, it would be very easy to make it an invasion of the US by almost anyone or anything. The point of the movie isn't aliens.
There's really only one character, a marine Staff Sergeant on the cusp of retirement, haunted by the loss of some of his men during his last combat. In the crisis of a sudden invasion, all hands are 'on deck' and the Staff Sergeant must keep alive a squad who doesn't trust him at all, proving to them - or himself - that he keeps to the marine motto of leaving no man behind.
That primary plotline didn't work for me. The distrust of the marines (and the coincidence of the presence of one soldier in particular) seems completely out of place in a story which is so monumental, so world-changing. The focus on "can we trust our (extremely competent and heroic) staff sergeant" sat badly with a 'rout of mankind' story.
Unfortunately the rout of mankind story was pushed to the background by the marine trust issues, and while it followed a not unsatisfying course, it did leave me wondering what real marines would think of these movie marines. The ones who walk into a 'hot zone' talking loudly. Who don't seem to have thought about snipers on roofs until after the shots rain down. Would the military send a squad deep into compromised territory to recover a small number of civilians when an entire mega-city is being evacuated and bombs are about to rain down? I found it difficult to believe in these marines (who contrasted badly with another group of SF marines, also led by a this-is-my-first-mission lieutenant). The real focus of the SF story is/should have been the gathering of intel, of quickly and effectively finding enemy weak points. That has been one of the main thrusts of most alien invasion movies because it is a very true one - our greatest weakness in such a situation is our ignorance. And "Battle: Los Angeles" does that as well, but it's not what the story is about.