I'm a huge sci-fi fan (especially of old '50s flicks) and I love the concept of this game. The graphics and especially the soundtrack are terrific, but to me, the game play is a disappointment. It's fun to zap the helpless humans and blow things up with the saucer, but after playing for a bit you'll realize it's pretty shallow and the missions are redundant.
I would definitely rent first before buying, or wait until a price drop. I can't see this being worth $50.