Buy Ikaruga. It's the best shootemup you can find in XBOXLIVE Arcade.
I tried the demo of PowerUp and I think it's a bit generic, with a classic gameplay of double thumbstick to move and shoot. I prefer GeometryWars because it has the same formula but is more challenging.
Anyway, I don't think that Powerup is a bad game, but it lacks of originality. Maybe the complete game offers a more innovative experience, but this is something that I don't know....