So, I finally finished this (well, beat the game -- I still haven't nabbed all the collectibles and I'm not sure I will at this point) last night.
Overall, it's a solid game, although it loses steam towards the end.
For those complaining that the game is too easy on PC, I've actually found one segment -- ironically, the final combat segment -- that proved otherwise. In fact, the game takes the obverse effect. The easier the level you play it on, the harder the sequence is.
The final battle is you in a helicopter trying to escape from an area. It's a rail shooter segment where you have to kill enemies to keep the helicopter alive. Most of the enemies are just firing small arms at you, so it's not that difficult to survive them. There's one or two mounted machineguns that fire at you, which represent a bit more of a threat, but they're pretty easily dispatched.
The real difficulty comes from guys with rocket launchers. THOSE will fuck your shit up badly -- if you're playing on the easy levels. At the harder levels, they're relatively simple to survive. The trick is to shoot at a lot of stuff on the airstrip which will explode. If you hit these exploding items soon enough in the chopper's flight path, you'll kill the rocket launcher guys before they can kill you. HOWEVER at the easier levels, there are two problems on PC:
1.) The rocket launcher guys spawn far in advance of you -- too far to aim your gun at them or anything near them. Thus, you get hit by rockets before you can fire back.
2.) The chopper your flying IS an enemy chopper, rather than a specially-coded "hero" chopper. Thus, it takes damage the same as any other enemy chopper. What's that mean? Well, at easier levels, it means that the thing is REALLY easy to kill.
So, bottom line, if you want to beat the last mission and have it be a bit easier, jack the difficulty up to hard or extreme just as you get into the chopper. If you want it to be impossible to beat, set the difficulty to medium or below.
I went through about 5 tries at this thing before I figured there must be some kind of bug and started searching around online. I saw the bit about the chopper being an enemy chopper and it all made sense. I increased the difficulty to hard or extreme (I forget which), and made it through the first time after that.
Just a tip if you get to that point and are wanting to put your fist through your monitor in frustration.