The realtime traffic info is ok in my opinion. I don't really have much to compare against besides the iPhone Maps. There's grey arrows on the highways where there's backup. I live in the San Fran bay area so this is fairly accurate. That's pretty good when I'm stuck in traffic, I have a good idea of where it ends.
There's also markers that you can click on and it'll say what's the issue. Sometimes it says accident other times it says slow traffic. Personally, I don't find that part too helpful. Honestly what I do I see the traffic, look on the iDrive and then switch to the news radio for what's happening.
I can't comment on the iPhone 5. I still have an iPhone 4 and I don't want to update the OS to iOS 6 yet.