I found this game both entertaining and frustating. Although I eventually made it to wave 5 and beyond (after a lot of trying), it felt like a battle against the odds: at some point, you've figured out the main mechanics and the controls but zombies just spawn way too quickly and way too many. Also, the shop is likely to offer upgrades that you can't afford (especially in the beginning), which might be intended but to me just felt gruesome and daunting knowing that you stand little chance without them.
Nevertheless, I think the game's got a lot of potential and I still had fun with it :) Maybe you should consider adding a feature where some upgrades are kept after failing.