Ahh yes, i'm glad you brought this game up Jeku.. I've tried it myself and it's a VERY addicting game. The funny thing is, as soon as i ran it it reminded me alot of Duo.. The gameplay is very different, enemies actually are attracted to you.. The circles work as the -black holes- of the game, once you shoot them they start up their pulling powers and suck in enemies(which can be useful, but obviously also deadly).
The game has fantastic gameplay though, when you shoot things it's like fireworks jump across the screen. You get the feeling that you're losing control, but if you make a mistake you feel as if, had you gone about the situation in a different way you would've been able to get through it.
I highly recommend this game to everyone, it doesnt just look good.. It is good, and definately worth the $5 price tag.
WindowsXP Home(Service pack 2), Athlon XP 2400+(2.01Ghz), 1GIG Ram, Ati Radeon 9800Pro 128MB.