I can't say that I've used it much, but the traffic data doesn't seem to be as full in terms of coverage as Google Maps. I just did a side by side comparison of traffic in both Google and Apple Maps:

Apple is reporting heavy traffic only on Flatbush, Atlantic, and the BQE (major traffic arteries for those not familiar with Brooklyn).

Google is reporting what Apple has, plus traffic patterns on a lot of other local avenues (3rd, 4th, 5th, 7th, Vanderbilt, Fulton, Union St, etc).

I believe they get their updates from TomTom. The routing around traffic was actually a bit better than Google's in San Francisco the few times I used it side-by-side.