This blogpost - although the text is non-english - shows clear comparison photos from New York City matched with resembling screenshots from GTA IV.
The images are coupled in pairs of two, the top image in each set being from real life New York City, the bottom image from GTA IV's Liberty City.
It's an interesting contrapoint to the rather negative blogpost (http://multiplayerblog.mtv.com/2008/05/05/8-things-grand-theft-auto-iv-gets-wrong-about-new-york-city-including-bagels/) at MTV Multiplayer, where a resident New Yorker felt that Liberty City misrepresents "his city".
The photos and matching ingame images clearly show how much feeling Rockstar have had with recreating a virtual New York City - however slightly twisted or not.
