It's a unique way to turn a ridiculous FPS franchise into a fun RPG, but it's "seriously" lacking.
Some weapons are scripted pickups, others are random, acquired from finishing encounters. This right here is really the only good replay-ability feature that this game has in my opinion. You may go through the game only ever getting one minigun, or one rocket-launcher, or one canon. The next play-through you may find 3 rocket-launchers or canons, and that can make things more interesting.
There's random item drops too, which can either really help you or really screw you. And this is a bad thing; there were a number of times where I could not pass an encounter due to lack of items or decent weapons, against too many enemies. This pretty much goes to show that some encounter/enemies require a very specific set of weapons to efficiently deal with, which isn't necessarily a bad thing, but can be rather annoying. Even though nearly every encounter is completely random as far as the types and number of enemies you face, the difficulty itself is really random too, which again can be quite annoying.
My biggest flaw with this game is that it's way too short. It can be finished in under an hour. There is an Endless Mode that unlocks after you finish the final boss, but the difficulty is too steep at the beginning so I've yet to even consider investing time into it.
Overall, it's still a really unique and fun game so I won't give it a bad review. However because it's so short, and the difficulty is bipolar as hell, it's not going to receive a great review either.