This game is amazing and never gets old.
GTA 4 has something for everyone. Its a shooting game, an rpg, a racing game, a fighting game, and open world sandbox, it has everything you could possibly want in a game.
It is also worth noting that this game looks amazing. The amount of detail in such a huge area (the city is enormous and i still get lost sometimes) is crazy.
The gameplay is also spot on. Shooting, taking cover, driving, etc... just feel right. The map and waypoint system is also great.
Usually in GTA games i dont care about the story or missions or anything, but i actually really enjoyed going through all the main story missions. With that said, you can never really get tired of just exploring Liberty City, driving around, shooting people. This game never gets old.
It goes without saying that this is a masterpiece.