I never really got into Waze; once I get to know my way around a city I'm inclined to just get around the oldschool way, sticking to arterial roads and all that.

I also used to work for a company where I occasionally visited an office where Waze was really popular with the folks who worked there. And found that that, when we were meeting up somewhere halfway across town after work, I would typically get there before the Waze users about as often as they got there before me. So, not really much of a time saver. Judging from my experiences on the occasions that someone gave me a ride somewhere, I was probably having a more pleasant, albeit lower-tech, driving experience, too.

If I had to hazard a guess, I'd imagine Waze has a law of large numbers problem: The sample size they have for estimating the travel time along any given stretch of side road is smaller, and there's also a lot more ways to route oneself on the side roads, and those two factors compound to mean a relatively high chance that at least one of the routes Waze considers has been assigned a grossly over-optimistic travel time estimate.