If you ignore the time you spend on special effects, how much longer does it take to do a decent job on an alien-invasion film? A script that makes sense -- how much longer for that? How much longer does it take to find better actors, or a least get decent performances from the actors you've got? How about getting the shooting permits so your entire film doesn't need to be shot in one building? If you put that all together how much longer does it take to do a decent job?
As it turns out, that question is easily answered. Brothers Greg and Colin Strause were hired by Sony Pictures to do some of the flying-squid-monster special effects for Battle LA while, at more or less the same time, they were also writing and directing their own, oddly similar, flying-squid-monsters attacking Los Angeles film called Skyline. Sony made noises about a lawsuit but lost interest when Skyline was such a dud at the box office.
It seems a reasonable assumption that the two productions were more or less in sync at some point and that the difference in release dates -- November 12th 2010 for Skyline vs March 11th 2011 for Battle LA (four months less one day) -- is mostly explained by the extra time needed to release an A-list vs a B-list production.
I won't say much about Battle LA except to note that it is a decent film and you should see it. I also won't say much about Skyline except to note that it is not particularly good and you might consider giving it a miss. Both of the films have decent special effects -- more or less the same effects, as it happens -- and if you've seen Battle LA then you don't need to see Skyline.
Since I've suggested that you skip Skyline you might want to know what it is about. So, a brief summary: A fashionable and well-to-do young black film maker has invited some friends from New York to visit. He is hoping to lure one of them -- a graphic artist -- to move to Los Angeles and work for him but the artist's secretly-pregnant girlfriend is not keen on the idea. A party is thrown in their honor in the film maker's penthouse. We meet the building manager who shows up to report noise complaints. He turns out to be one of the less tedious characters. Shortly after the party ends, while everyone is sleeping off the effects, giant flying squid monsters come thundering down from the sky and the special effects commence. In addition to the standard brain-eating toothiness -- and the ability to shoot sticky tentacles out of various body parts -- the main weapon of the squid monsters is their bright blue light which it is not good to look into because it 1) makes the skin around your eyes turn black like a raccoon's mask and 2) it takes over your brain. While they are not too busy having their brains eaten one-by-one, or staring into the blue light, the characters have a spirited debate about whether it makes more sense to hide from the giant flying squid monsters in an all-glass penthouse with the blinds closed or to try to break out and get to the marina because, umm... well, they seemed to think that being on a boat might... help... somehow... because, er, you know, maybe the flying squid monsters don't like water, or something. In any event, the whole escape-on-a-boat thing never happens because every time they try to leave the building they are turned back by drippy, brain-hungry monsters who demand to see their County of Los Angeles Fire Department location filming permits for the marina as required by section 22.56.1925 of the Los Angeles County Code. In the end (spoiler alert) only the graphic artist and his girlfriend are left -- she because giant flying squid aliens don't eat the brains of pregnant women, and he because there is something about his brain that doesn't agree with them.