Good game, huge problem
You have to start over every time. It sucks. There is no option to respawn with your existing upgrades and try again. It gets especially sucky on the first level with the tanks when certain areas of the level are totally saturated with weaponfire and it's impossible to avoid all of it much less most of it.
So you die after like 4 levels and have to start on level one. In my opinion, that's a bad design. Not even commercial games are that anal.
Other than that, really fun, but that thing makes it very much too frustrating.