We have the 7710 model. The complaint I had right away is that I couldn't see the screen well in the daylight due to glare. When I complained about it they sent me one of those clip on thingys that go on the top of the unit to reduce glare. At first they wanted to charge me $20 for it, but ended up sending it for free.
The first long trip we used it to travel back home to Texas, on one stretch of road the little car icon kept jumping off the side of the roadway giving me commands on how to return to my route even though I was already on it. And it was an old, well traveled road not new construction.
Then it routed me off the interstate and took me on a series of two lane farm roads into Amarillo instead of keeping me on the interstate. No clue why it did that, maybe had something to do with the settings as TusconJim mentioned.
Other that that, I am generally happy with the performance of the unit so far.