I hope you read this, I believe that it will help you and future games that you create. I played this for about an hour and thought it was good enough to give you a review. I really like the idea. You incorporated a story that isn't too deep, yet still entertaining. At first I hated the controls and was about to tear up this game in the review, but I saw that you had an upgrade for it so it was perfect.
Nifty story that doesn't go too deep
Fun shooter with plenty of enemies
Upgrades that are entertaining and enjoyable (and easy to use)
I liked the background in each level too. Also, the bosses were a nice touch.
enemies come from both sides, it adds an element of challenge.
It was not easy enough to get boring and not too challenging for people to just give up. I thought the difficulty was pretty fair (except for the last few levels).
CONS: (Don't be alarmed by such a large list, it is merely to help you)
No music. Please find some music to put to this, but remember not to make it annoying after 10 minutes.
The last levels were horrible to play. You are forced to have slow motion on the whole time or else you die in a few seconds. That part I hated. I wish your slow motion was not as strong because it was so dreadfully slow that I hated it. It is a cool idea, but maybe put less enemies in there and instead of being able to have slow motion on as long as you want, incorporate a bar that you use up when using slow motion and it will regenerate over time. That way you only use it in vital parts of the level.
I thought homing missiles were pretty bad for how expensive they were. Any gun upgrade dominates compared to it and so do the color changes (I am assuming they made you do more damage because that is what I thought).
More description. I did not really know the controls (use a mouse and click, duh). I was unsure if there were any specials or anything. I thought I was missing something whenever I saw a cow. Same with the upgrades, if you could just tell a little about them it would help (with the color change especially).
You don't show what level you are on. I had no idea if I was close to beating the game or not. It just happened. A little indication would be nice.
You could not customize your ship. This is not really a con, just more of a suggestion.
Sometimes it was really had to see gears (money) that was close to the ground because of the background. Maybe have the money auto-collect instead of you having to be near it.
The story was nifty, but it did not really follow up on each level. Sure there was a different scenario each time, but nothing led to the next. For example, if level 6 had some theme and then level 7 also kept that theme but with a slight variation. This is a popular technique that is used so you don't have to make new levels completely. You only have to change a few aspects of the previous level (level 6 in this example) and it will be a whole new level that had a combining storyline which is great (that would be level 7). If you keep doing this you can make a huge game with just a few different ideas or concepts.
I thought it was painful only picking up 2 pieces of money at a time from enemies. If you could have enemies drop more and increase the price of everything in the shop along with it, then it would seem like the money you are getting is actually valuable. It is almost painful to pick up 2 coins. I would rather pick up like 90 even if it was still just a small percentage of what I needed (psychologically it is human nature to want more. You can take classes to find this out, or take my word for it).
I wish there were more upgrades in the shop. Stronger gun, more bullets, maybe little droids to help you fight, faster homing missiles, stronger homing missiles. More would have been nice.
If you could put in special moves that would be nice. Similar to how I suggested you do the slow motion upgrade. If you could do that with a shield or a super cannon it would be nifty.
All in all, this is a great game. Good job and I look forward to more games.