I thoroughly enjoyed this game.
First, the looks and feel of the game were quite amazing. The tilt on the back of the screen coupled with the fantastic soundtrack immersed you into it. This was especially true in the higher difficulties as I started to sway and tilt with the screen accordingly.
The game play was also great, as it sported weapons with different feels and made you think how to properly use them. The ranking system could be argued pointless, but I saw it more as a way to see how far you came along from the start of the game. As for the upgrade system it was a good direction to go for a game such as this. As this is really a simple game, an extensive upgrade system beyond what is in place would have felt petty and a nuisance. The only thing I would have liked to have seen was something a little more for the boss battles. Maybe have on of the portals be the boss instead or something on screen to focus your attacks on while trying to juggle and prioritize incoming enemies.
All in all, a solid game that many people will come to enjoy for it's simplicity, graphics, music, and growing challenge as you advance in the ranks. Keep up the good work!