I recently setup OSMAnd to use the Google traffic as an overlay and tested it out over a 100 mile / 160km trip.
As stated before and elsewhere, it's just an overlay that OSMAnd is displaying, there's no intelligence behind it and it has no effect on routing decisions.
Initially, after plotting my route, I found it useful. I could see where the traffic hot-spots were so was able to make an informed decision as to whether to route around that area or not (it helped that I was somewhat familiar with the route I was taking).
Once I started the trip though, the usefulness dramatically decreased.
For a start, I set the overlay's expiry period to 5 minutes, thinking that would be enough to get periodic updates. But as I started driving, and OSMAnd zoomed in to show the road ahead, only the old tile was visible at the previous zoom level, presumably because it had not expired. So I had fat blurry roads, upon which my route was drawn in high clarity.
Another issue was getting the transparency level correct. At one extreme, you don't get to see the overlay, so that's no good. In the mid point, you get labels from both the base level and from the overlay, and they're often for the same road, and they intermix and it looks confused. At the full visibility, you just get Google's tiles/info and again, with a long expiry period, these are not updated anywhere near as often.
Finally for me, there was the issue of essentially having no warning. OSMAnd is getting the tiles for your position and a set zoom level. It shows the road ahead for a finite distance depending your speed. Even if there were a total road blockage, you're not going to know about it until you're practically there. For it to work meaningfully, OSMAnd would need to get the traffic data for the road ahead and have the ability to raise an alert. Just having a tile with a road of red traffic isn't enough, unless a passenger was constantly scrolling ahead to check the roads.
In conclusion, I think I'll use the traffic overlay for checking the journey just before I leave and potentially checking again later in the journey if I'm on a long one and taking a break. I won't bother using the overlay for the live journey.