Far too easy.
While I suppose this may not apply to all, I found this game to be of such small difficulty that I beat it within half an hour. I had most of the upgrades by the middle of chapter two, and with the Pulse Laser, I was able to breeze through every level with absolutely no major problems, and I only died of my own volition throughout because I missed a certain part of the level.
The upgrades are far too easy to obtain. By the end of the game, I had over four thousand dollars leftover after buying them all. For that matter, I'm almost certain that I could complete this game without buying a single upgrade or new weapon and only face a small increase in difficulty, meaning the upgrades make an already easy game a tedious task.
The graphics were standard, I saw little different from standard Flixel graphics, though the designs of some of the monsters were rather nice. There really isn't much else to say about them, honestly.
All in all, this is a standard game, which can be enjoyed for a brief period to kill some time or boredom. It is not of amazing quality, but I wouldn't call it bad. It is decent, nothing more.