It's quite good fun for a while actually, It gets adictive because you start trying to create as much roads to update the map as possible (Various routes to work etc). The more people in an area that use it the better it gets (Really quick road traffic updates etc)
Currently, it sucks as a Sat Nav, but I wouldn't say the UK map is even 25% complete, but it's certainly growing quite quickly and I can see this becomming a popular app. Privacy settings etc are there to hide your location from the joe public (You can usually see other users on the map) but year the wze servers will know where you are and thats how it generates traffic reports. - It's also intended to be used to advise other road users of speed traps
Good fun, but nothing more than a gadget at this stage! - it should evetually get to the point where it's a half decent sat nav with real time route updating based on congestion reports.