I had to help a friend of mine install some speed camera updates onto his Navman a while back (I can't remember which model it was; he's just replaced it now anyway). They appeared to install correctly, but in use, he found that the updates caused problems with the map data. He didn't give details, but said that he had to remove the updates to get the device to work properly.
He asked for my help again today when the latest maps for his new Navman wouldn't download from the CDs to the device itself. We couldn't work out a cure and had to load the old maps back on.