As with any mobile app, you're at the mercy of your wireless connection. While out walking in midtown Manhattan, I lost my cellular signal several times, putting a stop to my little blue dot's progress or sending it way off the street I was traveling down.
I knew where I was going, but if you're a tourist in an unfamiliar city, this could be a problem. That's the case whether you're using Google's or Apple's product.
The one clear advantage that Apple has is style. Like Apple devices, the maps are clean and clear and have a fun, pretty element to them, especially in 3-D. But when it comes down to depth and information, Google still reigns superior and will no doubt be welcomed back by its fans.