I've just finished the story mode of GTA IV. The last mission is pretty difficult, but not considerably harder than anything else I encountered.
It took me 35 hours to finish the 94 mission story-mode campaign. I'm still only at 60% game completion. And you know what the best thing is? I'm keen to get back and continue playing. I assumed that once I had finished all the story missions, I'd have exhausted my love for the game; but no, I'm itching to slip back into Niko's shoes.
This is one unmissable game.