I've got the Kenwood ddx514- pretty good head unit. Not too impressed with the user interface but then again I am not impressed with most of the interfaces from the DVD players. Ipod interface works great with my iphone but again, interface not that great.
I also bought the bluetooth add-on which works pretty good. Biggest complaint is that you can't disable bluetooth audio so if I hook up the iphone cable before the bluetooth connects, the bluetooth steals my audio. Simple fix is to just wait until the bluetooth connects and then hook up the iphone cable.
Kenwood is nice that you only need a negative wire for the parking brake (ie use a switch on the dash instead of using the parking brake). Makes it real easy to do scene selections or control the DVD while driving down the road.