Know a few people who have gone the Alpine route. Nav is far better than BMW but they also installed the Alpine HiFi stuff. The Nav screen retracts into the dash which is quite neat but not sure how well it integrates with the buttons etc
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2006 750i Sport, Monaco Blue, Cream Beige Pearl leather
I've been thinking about using a PDA with a GPS and navigation SW but really don't know if that will suit my needs well.
I'm still considering that as an alternative.
As I recall there already are some SW that plot your history, I'll let you know I find something useful in my quest for the Holy Navigation system.
Kenwood, VDO/Dayton (Blaupunkt), Pioneer, Alpine, Philips CARIN, Panasonic all make navigation+in-car entertainment solutions. I was at a car show last weekend ad saw them (even have brochures). I know that AC Schnitzer is promoting VDO/Dayton. I've seen a full room dedicated for that brand in their headquarters in Aachen (which I'm visiting tomorrow for another business). You should be able to use your existing amp. and speakers. For the steering wheel controls, I don't know. You may need to buy an in between interface but should work at least for the newer products.
I would like to hear from people who had done it as well.
Sorry, but I've read your request after I returned from Aachen.
They must have (proprietary?) converter to make their VDO/Dayton based solution to work with the steering controls. It would be stupid otherwise. May be you should send them an e-mail.
I'm little into the car stereo's and checked the matter with my installer who is really an expert. He claimed that there weren't any commercially available solutions. He recommends buying the BMW Nav system with the simplest radio and adding amp(s) and speakers after that.
On a different note, I've seen the Schnitzer S3 Sport which is based on the M3 with Schnitzer's aerodynamic, styling and suspension changes etc. It's soo good looking in flesh! Much better then the pictures in the brochure. Why can't the Alpina do something like that?
I've also picked up a 1/18 M3 Coupe model in Titan Silver.
I've found ot that the only aftermarket Navi that can be controlled with steering controls is Alpine. However, BMW has an aftermarket solution that is installed in the center console righ in front of the gear shifter, this is a VDO/Dayton navi so it should be possible to find an interface that works even for the ACS solution.
I've emailed ACS and asked, still no answer though.
The S3 Sport does look cool IRL, looks like a real racer, a little too much for me though.
Aftermarket Navi?
Much better then the pictures in the brochure. Why can't the Alpina do something like that? 
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