This game seems to get a lot of flack, but I have to say that I've enjoyed playing it. True, it's not really deep, but it's not quite as shallow as it seems at first either.
You *can* just play it as a 2D arcade space shooter. And it *works* as such. But if you try to play out the missions, and advance your race, and try to play a more involved game ... you'll suddenly find that five hours have gone by and your still playing.
Armada is certainly not among the Dreamcast's top games. It's not the first game that I reach for. But still, it's good, solid fun. You can probably find it cheap. It's worth it.