I finally finished the game on medium difficulty yesterday. Took me about 3 weeks to get through it, playing about 2 hours 3 or 4 times a week. It was by far one of the best FPS games I've ever played based on the graphics, environments, AI, weapons, etc. I had a blast and I'm a bit depressed that my journey is over.
Now that I'm done, I agree with others that the ending was very disappointing but it didn't take away from my overall experience. And with that said, the last stage (Volcano) wasn't nearly as difficult as some of the others. Based on some things I had read, I expected to really struggle at the end but the conclusion was relatively easy once you got beyond the final leg of the Dam where all the fat boys are running around the fields of lava and you have to cross over it to get down to the final base. In my opinion, Rebellion was the toughest stage of the whole game. I spent more time getting through that map than any other one and definitely used more restarts there than anywhere else because of that firefight leading up to the Archives. And yes, I know many people got through it quick simply by running (ducking danger) but I played this game with the mindset of killing every single creature/mercenary along the way instead of avoiding them.
Will probably play some select mercenary-only maps on Realistic mode now just for kicks until D3 comes out.