I have the Z120BT in my '11 Sienna, and I have the 920BT in a VW.
I had an installer put them in, though they can be DIY, it's not that difficult. My installer just crafted a plastic trim piece that looks ok, he says he'll have a trim kit my the end of November.
DVD options are not that expensive, you can get flip-downs for 100-200 easy. I have a NESA flip-down, it's 11.2in, looks pretty good. I think the dual-screen thing is ridiculous, and the nav/dvd are crazy expensive from Toyota anyway.
First the differences between the two. The 920 does not have dual-zone(movie in back, music in front, etc.). 920 screen is obviously smaller. Otherwise, they are really, really similar. I find the 920 a bit easier to use - the screen sensitivity seems better, and the larger fixed side buttons are convenient. The units even have the same user manual, you can download it from Pioneer's website.
So the good and bad of the Pioneers (it's not all good). They have the best iPod/iPhone integration, really easy to use, etc., more or less everyone agrees that the integration is better than on Kenwood, Alpine, etc.(with the additional iPod cable). And there is excellent Pandora integration - you control your iPhone's Pandora from the Pioneer unit. You can also play music from SD cards, and over bluetooth. Pioneer has always had good FM tuners. I find the tuner a little annoying though. The icons for preset stations display whatever the most recent RDS data was, instead of station name/frequency. Really odd choice. And reviews for the HD tuner add-on were so bad that I didn't get it. I don't have the XM/Sirius add-ons, as I think Pandora is way better(and free!) There's also an "AVIC feeds" app, that lets you input destinations on the Pioneer from your iPhone. Pretty cool.
Criticisms: Pioneer has integrated MSN direct, but they are shutting down service in Jan 2012, and I'm not sure you can even get the MSN antenna any more. It also has a built-in link to Goog-411, which is also being halted. Not big deals in either case, if you have a smartphone of any sort. There are some complaints on the forums about Pioneer's bluetooth microphone, but mine have been fine. Also, Pioneer navigation isn't perfect. Occasionally, it'll have me one street over from where I am, and it always has my road position a bit behind where I actually am (I think that's by design though). Kenwood uses Garmin nav, which is the bee's knees. Anyway, I liked the Z120 enough to buy the X920 a month or two later. Once the trim kit arrives, it'll be perfect.
Hope that helps. (How's that for a concise first post on the board?)