Addictive. Challenging. Inspirational.
I've never had so much fun with a Flash game! There are some truly visionary and inspirational games on NG, and this is by far the best Shmup the site has to offer.
The ability to turn your guns manually alone could revolutionize the genre, but all the other customization options take that even further. I found myself changing my entire loadout to fit the specific needs of certain levels, so even a gun as overpowered as the Rubber Cannon can't carry you through the whole game.
My only comment as to how to improve the game would be to add more unique customization to shields and engines. This game totally nailed the weapons, but something like a really fast engine that has a "slippery" acceleration/deceleration would be cool and balanced, or a weak shield that regenerates. Also, the Ship Mods should make the ship look different.
Overall, one of the best submissions I've seen to the website!
Did I mention the visuals are beautiful?