Great game... but it gets boring...
It suffers from the same problem as Assassin's Creed. It's fun initially, but repetitive gameplay makes it boring.
Plus the sniper is rigged, just save up for that asap, buy him and he'll get you by the first 10 waves without you having to do a thing.
Then you save up the money you earned from those waves and then you have enough for a rifle! Save up again and then buy the best gun, get all it's upgrades and repair+sniper your way till level 30ish. Then best gun+upgrades with repair+air support every round will allow you to never lose.
I got up till wave ~40 and got bored, died on purpose and got a score of 90k.
Perhaps ramp it up in difficulty or have sniper increase in power exponentially based on the wave?