Well I only use Android systems (phone sized or tablet sized) for my navigation.
A system like the T665 allows you to use any Android app, and rest assured, there are apps for everything.
For city / road ride I do use Google Maps or Waze, I consider them to be the best, specially when used online due to updated traffic information, radars, easy location and place search, etc
For Off-Road I currently only use DMD2 (which is included obviously), slowly even for road use, when im touring I tend to stay on DMD using the Routing > Road Fun profile. I would say the only thing missing (for now) is the ability to do multi-point route calculation, this is being worked on and should be available in 1 month from now.