yupe, this is what I'm doing, only 1499 for the pioneer + 250 for bezel total 1749 installed by myself :) with a lot better navigation system than OEM with DVD and CD on dash

which pioneer model did you buy? and also is it true w. aftermarket navigation that none of the buttons/functions on the steering wheel won't work?

FGI -

There are interfaces avaiable that will let you utilized the volume controls on your steering wheel. Anything is possible as far as steering wheel controls with $$$.

The most common unit would be from PAC.

I've been researching this major choice as well, and agree that there are some very powerful aftermarket options that quite easily rival and often exceed OEM's specs. The differentiating features will be plus-es for some and irrelevant for others.
For example, the Pioneer Z1, when it comes available, will offer:

Bluetooth phone integration (including Nav & handsfree calling to any of your contacts),
Mute of car speakers for all Nav Instructions & Calls
Voice commands,
XM Radio,
XM Traffic syncs,
30GB HD drive that can burn tracks direct from CDs onboard,
11 Million POIs (with phone #s if avail)
Intuitive Route- Learning
Onscreen DVD-playback (with workaround for while-driving if you want),
Touch-screen (this is one of my MAJOR issues with OEM- I've used touch-screen nav's for 3 years, and can't imagine going back to buttons),
AND steering wheel integration for stereo functions via a PAC module.

All of these features can be had with aftermarket at much less cost & headache than the OEM. BUT as far as I understand you'd lose the On-Board Computer with a double-DIN system, unless you do a creative work-around to install it elsewhere (like low center console), which may be more "custom" than some owners want.

For me, this loss of the OBC is the only thing holding me back from installing such a system. Probably not a deal-killer, but just something I'd like to test-drive or see in person before doing it on my X5. I'd even consider a single-DIN flip-up screen (like the N2, I believe) to avoid losing the OBC, but that's just me.
Hope this helps & I look forward to more conversation on the pros & cons. It'll help a lot of us make up our minds, I'm sure! Thanks for posting this!
M

What does it look like?
 

Can you take a picture so we can see how it looks?

I'm buying Pioneer AVIC-D1 and also the interface for stering wheel is only $50, regarding the OBC the new Pioneer unit has almost all function as the OBC so no need for it anymore. pics can be found here
http://www.mybimmer.net/x5doubledintrim.htm
the best of it is that CD and DVD is in dash with a clean look

I should have bought that from Roc3b when I had the chance! He was offering such a good deal for them but I made the call to late =(

Anyways will a different type of bazel work besides that one? like this..
http://www.mybimmer.net/CarPCLCD.htm or this http://www.mybimmer.net/doubledintrim.htm

That E46 bezel looks great and nicely integrated into the dash.The way that there is a bevel edge down to the screen makes the look all the more fitted as oposed to an after thought.I wonder if they make one for the X.
That is the best non OEM solution I have seen.

those are for e46 only, they wont work for x5's, the one I showed you is the one for the x5 i think it looks damn clean for been aftermarket :)

what is obc?