My only suggestion having gone through it recently, is to test iPhone interfaces in person. They can vary a lot. From number of buttons to navigate folder structure to radio only GUI, iPhone only, or both. Also how often you'll be using apps other than iTunes for music (as that head units GUI isn't like to be used and you'd need iPhone control). Afraid there's a reason some of these electronic companies haven't come up with a successful DAP, and piggybacking on other companies GUI can make it even more convoluted. I wasn't looking for phone capabilities, but on the music front I avoided some of the higher-end choices as I preferred Sony's iPhone/iPod interface. After awhile I was using WunderRadio, This American Life, etc. apps often though so it became less an issue (as long as you can control from iPod/iPhone). One Sony negative, it will begin to play a playlist track automatically. There's no way to shut this off. So if I want to listen to a TAL show, I have to let iTunes take over, start to play, pause, then back out and launch the other app (while the first track continues to display on the head units display). And that was one of the better UIs!
Also keep in mind most (all?) won't support the iPad.
EDIT: Got my Sony CDX-GT740UI also from Crutchfield. They even have an occasional video showing UIs. For example: Sony CDX-GT640UI - Additional images and video