Here's a handful of pictures showing the difference between the normal iOS 6 satellite view and the 3D buildings gimmick, by looking at one of the transition lines. Using my iPad for this, but it's no different from the iPhone details.
The transition line here is obviously the south side of Oxford Street. While the 3D model is sharper from above than the satellite view, I'd say the details of the 3D version make it hard to get a good sense of the streets overall. The different levels of the roofs and the roads all kind of jumble together.

Zooming in a bit, and tilting by degrees:


And a bit of macho cock-swivelling for the ladies (feel free to cut and paste this load for yourselves, lads):



That's it, really. Not sure what else you're meant to do with it. And for the sake of that we've lost actually being able to navigate from A to B sufficiently using public transport.