One thing about this game is that one of it's main selling points 'Lots and Lots of Guns' is very true to the point of excess. It can be rather difficult and annoying to select the gun you use, and even then you really need to make use of the 4 weapon slots since what may be an awesome weapon against one enemy may be crap against another.
This is nice for people that want to make the game play the way they want them to, but for anyone that wants to just pick up and start shooting it may seem way, way, way too complicated, because it is.
There's a couple boss fights that are a bit excessively difficult, and the problem there is that, for some of them, there's no easy way to restock ammo since, when you die, you come back exactly as you were when you died, so if you ran out of ammo on one attempt, you may have too hoof it back to the start of the zone where the ammo vendors are, and it's possible that some of the enemys will have respawned by then.
The respawning is annoying, too, especially when it comes to areas you have to run through several times, though usually you do so on a vehicle and you can just roadkill them (Which is highly satisfying, I must say)
My only other complaint is the way most enemys spawn: A set of 2-4 'holes' where there will be 1-2 standing around near the holes. Once you kill those loitering enemys then new ones will start emerging from the holes one at a time. What happens here is that you spot these holes, go waaaay back with a sniper rifle, kill the ones outside, then just pick off the enemys as they emerge from the holes one at a time.
As a whole, it's flawed, but I'm having fun with it still.