If you commute more than 10 miles to work these days, you can't live without it. It provides you with live traffic info and alternate routes when people are rude enough to crash in front of you. Southern California is one giant gridlock for at least 6 hours a day. Waze gives you a chance to avoid a lot of unnecessary delays.
Best phone app, gives voice directions just like a standalone GPS/NAV system and linked to bing for internet searches for places for it to take you to, and its FREE.
Recently I had to drive from Reagan National Airport to ~ West of Baltimore one dark night in the heavy rain and fog,
that's lots of exits from highway to highway around DC where I am unfamiliar and I never missed a turn with waze.
I cant imagine how difficult it could have been.