i've been using waze for awhile - the best part, for me, is the tagging of hidden speed traps - i see the police coming up, and i can make the appropriate adjustment. obviously, sometimes it's "old information", but then you just delete it for those coming behind. i like the idea that my fellow wazers are a helpful community.
yes, as someone has mentioned, the map is sometimes inaccurate, but that's a work in-progress, as well. you can go to the map editor, and correct the one-way problems, and no turns allowed, etc.
my main goal was to find as real-time traffic information as possible, and i think waze is good for that. a lot of the other services (like the native google map) have a significant delay in traffic information, whereas, depending on how many other wazers are out there, the information is much more 'real-time'