It's ok, but I think it would be a lot better if you were able to shoot the full length of the screen. I mean if you're forced to wait until the enemies have already covered half the distance to you then you're in pretty bad shape if you have a weak gun. At least if you could shoot the whole length of the screen you could take out more of the baddies before they get to you.
Also, it would be nice to have some info on the guns: rate of fire, damage output, size of clip/number of bullets, reload time, ect. Maybe even have an option to upgrade your existing gun/guns to improve certain attributes.
One more thing...nothing infuriates a gamer more than a "you die, you start over again from scratch" type game. If you only implement one of my suggestions, then you better make it so you can continue from where you left off and keep all or at least half of the money--as well as the upgrades (if any)-- you had when you died.
I'm sorry for the low score...this has the potential to be a great game; it just needs a little work to get there.