Haha, my wife made me a "nerd" costume, with glasses and fake buck teeth, but that doesn't really come across in the small av thumbnail.
Anyways, I felt the same way about IV. All the new AI and physics was really cool (actually wrote a paper about the engines for a grad class), but WHERE IS MY FUCKING JET PACK!!!! That was like the biggest step back ever. Plus I could give a shit less about Niko. I played until I unlocked the 3rd island, then I just stopped and started doing random shit.
Free roam or whatever in online mode is fun if you get like-minded individuals playing with you. I once killed a chopper with a car. That made my day.