I've been using Organic Maps for almost 3 years. I lived in Chicago during this time, as well as some smaller American cities. I go back and forth between Organic Maps and GMaps depending on the situation.
I've found that Organic Maps' lack of traffic data isn't a big deal for me. It doesn't always give you an accurate ETA, sure, but it isn't any worse at actually getting you to your destination.
The thing with GMaps is that everyone has traffic data, so nobody has an advantage. Google's alternative routes end up equally saturated as the main routes, meaning a "dumb" maps app that always takes the main route will get you to your destination in basically the same amount of time. This is backed up by my own personal experience, and some academic research [1].
Now, when I do need an accurate ETA, I go back to GMaps. I'll also use GMaps to route to businesses sometimes, because OSM doesn't have up-to-date info about businesses throughout most of middle America.
Some places live traffic information gives you a choice between a 10 minute way and 40 minute way though, if you get stuck in the wrong spots it really truly sucks, and for us who live in these places, being able to easily route around those spots saves us a bunch of time and energy.
I want to use any client app that uses OSM for car navigation, as I contribute both money and map fixes, but currently nothing seems to come close to either Wave or Google Maps when it comes to traffic information, which ends up being pretty important (for some), so I end up using Organic Maps only for when I walk on foot.
Saved me a lot of speeding tickets on the interstate.