The newer units offer a lot more over the E7006 for example. The E7008 has a 4-disc changer built in, which also plays MP3, & WMA files. The E7006 doesn't. Also, the newer units have Bluetooth built in, and has a lot of phone functions which in my book, is a BIG selling point. The display & graphics are even better on the new ones.
Of course, with the new ones, they've made it so you can't use a lot of functions while the vehicle is in motion. The older units can be overridden to allow this.
Of course, the newer units will cost a bit more, even on Ebay, but I think it's worth it. If these features don't matter too much to you, you could probably get a great deal for the older 4th gen units on Ebay.