This was an enjoyable timewaster that reminded me of a cross between Centipede, Gauntlet (hordes!) and Robotron, and I played until everything was maxed out, and I had over $500K, before getting bored, cramped hands, and stopping.
One beef I did have was that, if the game was becoming too hectic, one could stop playing, let nature take its course, then Continue - with the chance to upgrade of course. Thus the upgrade part of the game became one of endurance rather than skill. Perhaps docking a percentage of money for a continue (or maybe wave# x $1000?)
Another is that in-game the FlashFooty logo is underneath the 'next' button, which made me nervous about possibly opening another website either in-game or when visiting the upgrade store. Just a little UI hiccup you might want to look into.
Finally, one lesson I've learned from games with upgrades is that you're upgrading for a reason beyond slaughter into infinity. Here... I didn't feel like I'd accomplished anything except being able to circle around and around the playfield turning square critters into red smears. Maybe a big boss that appears around wave #30 and lets you unlock endless mode, or perhaps integrating scoreboards or achievements?
Otherwise this game was a blast. Insane firepower. HUGE numbers of enemies onscreen didn't slow things down at all. Great fun!