I've got one but have managed to lose the instruction book!! D'oh!
Nevertheless, by trial and error it works OK. But it does seem to 'hold' direction instructions on the screen too long. (Maybe I can change this in the settings?).
The screen is small and loses detail if you want to use it in a passive non-navigating mode (i.e. you know where you're going but might want to use it to work out a back-double if there's been an incident). No experience of TTGs or similar, personally, but they are all getting a lot cheaper!
I use it in my Boxster, where the built in scamera locator is a real plus. Not as good as COMAND in my 270 but cheaper than fitting Porsche equivalent!!