Well, I want to like it, I really do, but there several things holding this game back.
1.) I have no idea what my hit box is. Is it a pixel at the end of my arrow? Is it my ship? Is it a box around my ship? If you don't know EXACTLY what you're trying to keep from touching stuff navigating patterns becomes simply a guessing game.
2.) Shields. You say the ships have them, in fact different ships seem to have different types, but you never explain how they work. As near as I can tell shields seem to be related both to how much damage the ship can take before losing health as well as how many 'lives' you have. What shield regeneration has to do with anything, though, I have been completely unable to figure out. Why would you add such a critical feature to a game as large as this and not bother to explain it so players will be able to use it!? It would have taken no more than a paragraph, or even a sentence, in the help section.
3.) Finally, the game is far too repetitive. I'm not asking for an in depth story line, or and story at all, but the difficulty progression is laughable. The first levels are too hard and, with the powerful upgrades available, the final levels are too easy. After just a few levels the repetition becomes tedious and it becomes more like hand-eye exercises than an actual game.
Essentially, this game is just lacking too much polishing to make it truly enjoyable. It is large, well built, etc. etc., but because you didn't bother to take care of the small things like making sure the bullets didn't blend into the first background and making the effect of upgrades clear this game is...frustratingly not as good as it should have been. The potential is obviously there, I wouldn't have bothered to write a review is I hadn't been impressed by aspects of this game, but as it stands this is just another bullet hell.