It's as much a cultural thing as it is a generational thing. In Argentina, where I grew up, showing up unannounced to a friend's house is normal (some people might add: "as long as you bring some food!"); on the other hand, in neighboring Brazil it was seen as a pretty rude.

So, I guess, what might not work for you might work for other people. I personally prefer at least a couple hours of heads up, but don't mind doing a last-minute dinner/lunch when I'm close to a friend I haven't seen in a while.