Fictional cities are great, too! You can create all the mayhem you want, and no one will weep after the landmark you've just wrecked ... :-)

But there's another side to the intimidation coin (apart from everybody in this bar being so bloody proficient at EVERYTHING they do ... ;-) ) and that's getting carried away. I've been to loads of places, and have to be really careful not to let the descriptions take over the plot. There's also a danger of the author's voice taking over that of the character they are trying to create ...