We've had both a Kenwood and currently, a Pioneer. I much preferred the Kenwood. It (at least the one we had) was Garmin based and worked very similar to my stand-alone GPS. Mostly small things that drive me crazy. For example, the Pioneer system we have doesn't display the current vehicle speed.
Personally, I wouldn't have another built-in navigation. As has been stated, the map updates are costly and in some cases, difficult to do. The built in system can't easily be moved. They are also typically much more expensive than a stand alone (obviously depending on what you buy) GPS. My $100 Garmin does everything I need it to; can easily be moved to other vehicles; and has free lifetime maps. My second choice would be using my cell phone with a built-in system being a distant third. The only reason we have the Pioneer is that it was already installed in the van we bought. If it dies, it will be replaced with a non-navigation system.