I am a huge Garmin fan and if you already own a GPS it is probably not worth it. However, when I get my Storm later this week I will probably try the VZ Navigator for a while and put the Garmin in my wife's car.
Why? Even though it is about $120 a year, map updates are included. Most GPS units update maps about every year and a half for $75 or more. In addition, if it works well, the convenience of only having one unit in the car might be worth it. Especially when renting cars when traveling....
So....I tried VZ Nav in the 8830 and it wasn't nearly as good as my Garmin Nuvi. However, one can hope with an update and the larger screen on the Storm it would start to become a compelling option.