I can't make any claims for all versions of Droid, or all versions of Sync, but here is what I have:

Droid running stock 2.0.1

2010 Ford Expedition with Sync 3.1 and Voice activated navigation system

I have been very happy with the overall interoperability.  I am able to download all of my contacts, place calls via voice command, see signal strength and battery remaining on the phone from the Nav system, and stream Blue Tooth audio (although BT control for audio is somewhat limited and a little bit quirky).  With BT audio streaming, the Sync system automatically starts playing a song as soon as the Droid connects (even is you don't have BT selected on the audio system) and the Sync/Nav system controls don't seem to work.  You have to access the music app on the phone and pause the song if you don't want it to keep streaming in the background.  Pausing the app on the phone also seems to then activate basic Sync control of play/pause, and next/previous track.  BT streaming volume also tends to be very low to start with and you have to bump the volume on the phone up to get it to acceptable input levels.

If I plug the phone into the car's USB port I am able to mount the SD card to the Sync system and have full control of the songs with the standard USB voice commands and Nav system buttons, plus it works well to keep the Droid charged.  You are still able to use all of the BT commands for the phone portion of the Droid even if you are using USB for the audio portion.

I know that I have seen some reports of various issues depending upon Sync version and/or original vs. updated Droid O/S, but so far I am very happy with my Droid/Sync experience.  There is still room for improvement, but both platforms are still under a fair amount of development so I expect that things will only get better with future releases from both companies (I know, this is probably wishful thinking given Ford's slow response to the iPhone 3.1.3 issues but it can't hurt to hope!).