I've had a whirl with the sat nav now. The largest issue here is the search functionality, and the way we've got used to functioning under Google; you chuck in the name of a place or a business, and it finds it. Apple's tries to do the same, but has two issues. First, it simply doesn't have the same number of addresses in the database, so a lot of things get missed. Second, it has a habit of thinking it's got the right match when it hasn't. You don't get a list of matches to select the correct one - it seems to either be a yes or no affair. In instances where there are multiple matches (as an example, if you make a generic search such as 'Starbucks'), it will put down pins for the 4 closest matches to your current location. I found myself reverting to either postcode lookup, or finding the location on the map and dropping a pin there myself.
Once it's going, it seems ok. When planning a longer journey (about 35 miles), it gave me three routes on an overview so I could pick the most suitable. I'm not sure if it adjusts for traffic or not: there are some red lines on the map when traffic builds up, but I didn't at any point get warned audibly. Perhaps it already caters for this on the route, so doesn't see the point.
Battery gets absolutely eaten when it's running. A 15 minute journey took it down 10%. Again, not a huge problem as you'd usually be charging in the car, but worth considering if you don't store a cable there and may find yourself wanting to use the sat nav on the fly. This is on the 4S; hopefully it's handled better on the 5.
My String Journey: http://tt.tennis-warehouse.com/showthread.php?t=362403