Probably closer to 3.5 stars, picked up mostly because of the movie coming out. This book appears to be something different than what the trailers for the movie tease, as this is a book with a clear, albeit somewhat ham-fisted, message to deliver.
The book gets right down to it - the aliens crash, and they're taking over. The timeframe is frontier times, so the Earth technology isn't great, but the battle happens regardless.
It's a quick, enjoyable read overall. Not perfect, but not terrible, and it might make a good movie regardless.