I have actually done exactly that. Have gotten to about 44 percent or something and suddenly just stopped wanting to go back and play it and have yet to do so for more than a few minutes. Heck the last time I was on it I turned it on, started shooting stuff in the apartment, somehow managed to get a 6 Star rating and ran out to the top of the stairs and hid there with the machine gun and killed anyone that came around the corner.
Easiest achievement, although,
I did turn it off after because I'm just no longer interested in the story, at all.
I think for me it might be because at this point, Niko does not have to do anything whatsoever, he's already got hundreds of thousands of dollars, he could just leave the city and use the money to start a better more carefre life away from the retarded threat of, oh nos, Russian mobsters. Meh, it's just too flimsy in logical reasons to stay even if there is somebody he's trying to find in the city, he never actively does it and it's boring as hell waiting for that to randomly pop up in one of the missions.
So, yeah, the story and missions lack staying power after a while and now I'm wanting to go back to playing Assassin's Creed even though I know how repetetive it can be. Hell, finding all the templars sounds more exciting than playing GTA 4 some more.
I'll probably sell it to my friend soon if I can't get back into it.
(Although I even noticed that I haven't been on XBox Live for any game in a month's time... maybe I'm just tired of playing games. :O)