I have been using a Garmin Nvi 765T since March. The core navigation features are very good.
The Bluetooth hands-free calling is... inadequate. Everyone I talked to complained of having a hard time hearing me. (Windows up, radio off, mic on my collar/seatbelt/steering column)
The FM transmitter is basically useless. This may be due to living in a metro area (Tampa), but our FM band does have some availability, and the Garmin never seemed to get thru. I just use the aux out on the Garmin to the MS3's aux input.
I don't like having obstrucitons above the dash, so windshield mount was never an option for me. I also didn't want to drill holes in the dash. At first, I stuck the Garmin mounting disc over the passenger air bag warning icon to the left of the shifter. but as I feared, it was a real tight squeeze to shift into first without punching the Nvi with my knuckles.
So I went with a seat bracket mount from MountGuys dot com. It's basically an 18" chunk of 1/2" flexible steel electrical conduit, painted black. It bolts to the passenger's seat rail, right next to the center stack, and has a recevier ball for the Nvi's powered quick-release adapter. It sticks up next to the center console on the passenger's side. Does not interfere with the passenger's comfort- very easy to avoid.
I'll see about snapping a couple of pics in the morning.
***If you buy a Garmin, DO NOT GET THE MSN DIRECT OPTION. MSN Direct is going bye-bye on 1/1/12.