A few tips for those starting out:
-Ignore the Machine Gun and Coin Magnet. The Machine Gun becomes very underpowered later in the game and the coin magnet isn't strong enough to make a serious investment in.
-Make the Cannon your prime weapon. The Cannon does the most damage in the quickest fashion
-Use the shotgun only when dealing with the small swarms of enemies.
-Don't make a serious investment in Rockets or Lasers until they are needed. You waste valuable money by investing too early while expecting some sort of payoff. Both weapons are only needed for a select number of enemies. Remember, your Cannon should be your primary weapon.
-Speed Boosts should only be bought after Income has been fully MAXED.
-Health should be bought in conjunction with Cannon upgrades and income upgrades. It is perhaps the most valuable perk to buy.
As for the review:
The game was decent. It gets old very quick and doesn't really deviate from the vast number of shooters. The upgrades were nice, but It didn't distract from the monotonous shooting and annoying music. The ending was a big disappointment. Perhaps a storyline would have made the game worth playing. It wasn't a bad game by any means, it just didn't really feel new or exciting.