I've used my 660 across Europe and in the US this year (came with Europe installed, I purchased a memory card and the North America maps). In the US I used it during our vacation in a rental car - it worked great except that some (very) small roads were missing.
Around Switzerland, Germany, Austria I never had problems. In Eastern Europe (e.g. Czech Rep or Hungary) the maps are surprisingly accurate, although there were 2-3 occasions where it led me through a road that was closed for cars & motorcycles. No big issue.
There are very few things I would like changed, e.g. some of the frequently used functions are hidden deep in the menu system. I also didn't find a way how to skip a waypoint. Beside of that I'm quite happy with the device.
I use Tyre to plan my trips. Tried also RouteConverter and MapSource, but Tyre works just fine for me.