It's a decent time waster for the first time, upgrading the weapons isn't really hard and once you've unlocked that it, wouldn't have minded a few color schemes for the weapons though (because later in the game you get to much money after all the unlocks and there's nothing to upgrade to so a color scheme would be nice) and all it is is zombies coming after you.
I'd introduce something like a harder to kill boss that you have to duck his swings or jump over his attacks maybe every 10 levels to add some excitement to it. It's not bad for a quick time waster though, but there's nothing to really pull me back to the game after playing it once and unlocking all the weapons and upgrades, wouldn't mind a few more weapons either, or maybe more play space, also would mind the action on the screen to be a bit further down instead of right in the middle but that just personal preference, it's not the worst flash game I've ever played especially since the first few minutes were actually quite enjoyable but it could go through some improvements, also the font is really hard to read since it's so small.