Here are my impressions after nearly 5 hours of playing:
I absolutely love it. I've always liked the GTA series, but GTA4 has places it among my favorites. Honestly, this has to be the most in depth game ever created. For a good 2 hours, I drove around and experimented with the physics, exploring new areas. I went through individual pages of the virtual web environment created by Rockstar (wow, the amont of fake web content is astonishing), and watched several episodes of TV in Nico's apartment (yes! Bas has his own show!). There's so much to do, and every aspect is oh so polished.
It's a lot different than the other GTA games, yet very similar. So far, the story seems like the biggest difference: it's incredibly engaging and the characters are awesome. The voice acting, during the vast majority of the cutscenes, rocks. The changes to the controls and the addition of the cover system greatly improve fire fights and maneuvering the city.
The atmosphere has a great deal of charm, too. The city looks incredible, and the shifts in weather really affect the mood of the setting. In full daylight, jaggies are present on some cars and building edges, but for the most part, they're not a distraction. I've been playing on the PS3 version, and I've only noticed pop-in when I use the cinematic camera- during some of the camera angles (such as a fast switch to a behind the car camera). These are the only instances, however, and I haven't noticed any while driving around or running. Frame rate has been really stable so far, too.