I don't believe for one minute that you and your friend drive around testing different sat navs against each other; why would anyone do that? But even on the offchance that you actually do, not everyone gets the same results. You will see that Tom Tom has been criticized in this thread.
I too have had bad results with Tom Tom.
Well, I have tested them thusly, and read extensive reviews by others that have, as well. The reason is to find out which is best. You couldn't guess that reason? And you all keep arguing 2 different topics interchangeably, which doesn't work. Let's separate them.
Maps:
NONE of them have perfect maps. I carry a paper map, Garmin standalone, and iPhone with many options for my delivery job. I have had destinations that were on zero of these maps and I had to figure it out myself once I got close. The newest map is not even always the best, but I do find online maps to be a bit more accurate more of the time, presumably because they are more quickly updated.
Routing:
TT is superior for its routing. This has little to do with maps, it is the routing software that is great, and importantly, adaptable. That said, I can find routing errors with all of them, as well. None of them are perfect for routing, either. Knocking just one for imperfection is silly, esp if it happens to have more success more of the time. Garmin claims to have a system similar to TT's IQ routing, now. I haven't tried that one, yet.
The routing is most important if you are using a gps frequently. If you use one only occasionally, random positive/negative results for either mapping or routing will occur and frankly, any such reports are useless. Seldom does a non-updated map affect me as much as a poor routing choice.