I've used Pioneer's i2000 interface and let me tell you it sucks!! It's nice cause it keeps it charged but you can't find anything on you're iPod through the head unit, you might as well forget searching for any songs and just make lots of playlists.
On the other hand, I have an Alpine head unit with their one-wire interface, and I can control it through the head unit just like I am using the iPod, nearly as quick and easy as using an iPod as I can use the volume knob to scroll.
I was looking to buy a new head unit cause I had a Pioneer and was not going to touch their iPod interface, so I starting searching the net and Alpine's iPod interface was at the top of everyone's list as far as consumer reports, best ratings, and was even best rated in online audio forum.
Can't go wrong with Alpine, hands down the best!