I have been enjoying the campaign and there are a lot of nice features in the game along with good physics and 3d responding water effects.
However there are a lot of things missing from San Andreas and Vice City that I miss a lot in the campaign. No parachutes, cant buy houses, cant buy or customize cars (even though they had game trailers showing you could buy cars), no RPG elements for Niko, no quads, no monster truck, no tanks, cant increase running stamina or any other items like that, story is quite short for a GTA game, and much more. Cant golf on the golf course, cant ride the rollercoaster. None of those epic moments like when you buy the hotel with a helicopter on top.
While some of that is disappointing the multiplaer truly makes up for it.
Here is what you can do in the GTA4 multiplayer.
16 players thru the entire city including helicopters, boats, bikes, cars, trucks, pedestrians, and cops. You can select individual islands to play on or you can play with every island open if you want. You can turn cops on or off, turn pedestrians on or off, set the amount of pedestrians or traffic on the street or none, put whatever weapons you want, set spawn time and spawn distance, time of day, weather, just about anything along with 15 total multiplayer game modes including some that are 4 player co-op missions. You can even set up online races with boats, cars, trucks, bikes, helicopters, police cars, SWAT trucks, and even firetrucks. lol I have hosted matches with the entire game open and also hosted matches with only shotguns at the prison. Also hosted matches with just rockets on. Lots and lots of options for sure and anywhere you want to play, you can. You can make the game setup as big or small as you want. I have even set up grenade only matches. lol

So overall I say the multiplayer is where this game truly shines and they have put a lot of effort into making that run real well online. The campaign is fun but just doesnt seem to have the excitement and fun that the other GTA games had.