The overall game is pretty fun once you get a feel for it, but respawning is too slow. It probably sounds lazy, but having to click every time you fail makes it feel a lot less fluid than if you just respawned instantly. That's part of the reason I loved the Boss Battle so much: Every failure immediately transitioned smoothly into the next attempt, which made the fight feel much more fast-paced and downright epic (Although the fact that you were a normal Grunt facing a Zombie Mag Agent is epic on its own).
The checkpoints sometimes feel a little too oddly placed. The section with the pressure plates and the laser scanner dragged on for a very long time before you got another checkpoint, while the one directly after with the locked doors I finished on my third try. If and when you make a third installment (And I hope you do), I'd suggest trying to average out the amount of clicks and amount of time each section takes so the level of difficulty feels a bit less random (Aside from Boss Battles, obviously, which should have more at stake to get the adrenaline pumping).
Overall, pretty good with some annoying bits, and I'm excited for more. 4 Stars.