I have a V735 and it is pretty good, however, some of the routes it chooses are a bit pants, no worse than Garmin, but still a bit odd.
I just don't like Garmins routing alogrythms, so I would never buy one, not for the UK anyway, in Europe and the States they seem fine, in fact really good, but here?? terrible.
Unfortunately I cracked the screen on my Mio V735, but if you do get one let me know and you can have all the accessories for it, like charger and cradle etc. so you can keep them in a second car.
I wouldn't bother wiring them in to the car, you will need it constantly on the aux input to hear them.
You could look at the Pioneer AVIC F500BT, this comes with an active bracket and mutes the stereo when you get nav instructions, it is also a nice big 6" screen and it has a Parrot Bluetooth system built in as well, which works really well.
This is it fitted to our 123d..
In the end though I put a Sony Nav-U in the OEM pop up housing....