Checking out my own home town of Chicago, I was a both impressed and disappointed by the completeness of the map. OnionMap only shows a few of the city's tallest historic buildings and some buildings are hidden behind taller ones because there doesn't seem to be a way to change the angle of the view. OnionMap does however give a surprisingly good overview of Chicago's downtown "El" train system and the 3-D view makes it easy to locate train stops.
But ultimately, all these features are fairly meager compared what you'd see on Google Maps. OnionMap's 3-D view gives you a good way of orienting yourself in relation to well-known landmarks. But Google's street view seems to make more sense as a tool for helping lost tourists and even locals find their way around.
All-in-all, OnionMap seems like a great idea whose time came and went a couple of years ago. Today with Google Maps and its competitors being so richly featured, it's hard to see where OnionMap fits in as anything more than a pretty toy. But it sure is pretty.