I've still got it, but dang, I've yet to finish it. Don't get me wrong, it's incredibly impressive--Liberty City itself is a living, breathing work of goddamn art. And the new level of maturity and semblance of realism exhibited by the series is really refreshing, and I think a good sign of the medium continuing to evolve.
But. It got really boring really fast. Taxi to get food and armour. Taxi to a mission hub. 10 minutes of chauffeuring and exposition. Obtuse, finicky cover-based shootout/timed, scripted vehicle chase with poor-(realistic!)-handling cars. 10 more minutes of driving and talking. Ignore a pointless minigame. Repeat.
On the other hand, while I don't respect it anywhere near as much as the venerable GTA series, I found Saints Row 2 to be a much more fun, satisfying game to play. It was crass and cartoony and relentlessly stupid, but goddamn it, it was fun. Getting into firefights with the local law enforcement while joyriding a helicopter in Grand Theft Auto is fun, but doing it with regenerating health, diverse weapons, and a character I built from scratch in Saints Row is awesome.
GTA IV was significant as all hell, but I didn't enjoy it anywhere near as much as, say, R*'s Bully or Red Dead Redemption. Or earlier GTAs for that matter.