I've been a fan of the GTA series since inception and this game is worthy of carrying on the name. If you've played GTA3, Vice City or San Andreas then you will know what to expect. GTAIV alone will take 30-45 hours to complete, more if you explore the map. The 2 expansions will take you 15-20 hours each. That's pretty good value.
If you can, get it from Steam. They often have deals (I picked it up for £6.22), so if you don't mind digital downloads it's the way to go. Also means you don't have as much updating to do once installed.
Story wise, it's a bit more in depth than it's counterparts, but the story isn't actually that interesting. However the missions more than make up for it. It is also nice that the 2 expansions follow the same time frame, so lead characters bump into each other on certainparts.
Map wise it suffers from being neither too big, nor too small. It is too built up so it's harder to navigate. San Andreas was bigger, but there was an openness to San Andreas which this game doens't have. Needless to say it takes a while to get used to where you are.
Music has always been a big part of the GTA games and GTAIV is no exception. On it's own it is very lacking. It is pretty much all modern rubbish or rap/dance/reggae stuff. Nothing mainstream and very little rock. However the expansions do bring extra tracks and stations which make it a little better (Vice City FM!) for older gamers.
All in all you can't really go wrong with this pack. It is just fun.