I find the OVi maps work really well, I used my TomTom and my 5800 at the same time, and it was just as accurate, there aren't as many features and options but as a back up it'sfine, I used to have TomTom mobile on my 6680, and n73 and I'd say it's on a par with that.
The main drawback is that free doesn't necerssarily mean free as there are data charges, which are high, I used the maps in the house and it cost me ?5 for five mins on 02 PAYG as it uses a lot of data, it has to go online to locate and plan the route and then download it, so until such time as it's fully self contained, it is going to struggle