PewPew is the best multidirectional shoot them up on Android. It's basically megatons of enemies and many different game modes, everything rolled up with sweet smooth retro graphics.
Win medals, unlock ships, and compete on the online ladder board.

There are five VERY different game modes:

*Pandemonium:
Some enemies explode when they die, and the other spin maniacally everywhere. Hide from the explosions or deftly go through them.

*Dodge this:
As you collect boxes, there are more and more enemies moving around. Naturally, you have to dodge them all.

*Assault:
You fight against waves of enemies. Spoiler: at the end, they win and you die.

*Chromatic Conflict (NSF color blinds):
Enemies of different color appear. You can only destroy the ones that are the same color as your ship.

*Asteroids:
Need I say more?

Features:
*High frame-rate (honestly, this is what defines this game)
*Finely tuned game-play (oh, and this too)
*Online leader boards
*Online replays
*Sweet music
*Unlockable ships.
*XPERIA PLAY Optimized
*Controller support for NVIDIA and Android set-top boxes.

Now a few notes:
-Kill as many background app you can, this can considerably increase the smoothness.
-Seriously, kill them all.
-On some devices (such as my Nexus One) the touch screen has trouble handling touches on the same horizontal or vertical axis. That is why by default the joysticks are in diagonal. You can try moving the joysticks to the bottom using the settings menu, but it might result in poor controls.
-No personal data is sold and there are no ads. I'm really sorry ads-lover.