I have the HK drive+play hooked into an aftermarket Pioneer head unit and it rocks. The only bug I have found in it is that sometimes it resets the main menu options (like what i want to show and dont want to show - genre, composer, etc) but that's not a big deal. I found one on ebay complete with a FM modulator (which sucks, like most FM modulators do - go with a direct cable input) for $130. The design of the screen and control knob are very sleek, looks well at home in the bimmer. One tough thing is getting the main unit behind the dash - its a little cramped back there. Running the wires so they are hidden well is a daunting task, but it can be done with patience. With the stock head unit, the only real option is the FM modulator - that is why I went ahead and installed my old head unit. Sad to say I lost the controls on my steering wheel, but it made up for it in awesome sound. I have some pics of the installation that I will post later.
Another ipod interface I used was the NeoIon. It was pretty cool too, but there wasnt any way to see what song you were playing, you pretty much had to use the first 5 playlists, or go to an all song playlist. and with 4000+ songs on my ipod, that was too much of a pain in the a$$. It was very easy to install, however, hooks right into the CD changer controller. You do lose the ability to use your cd changer with that unit. I ended up selling it on ebay for just a little less than what I payed for it.
If you have any specific questions about ipod interfaces, let me know, I researched a bunch of them and am very happy with the drive+play.
oh BTW - the NeoIon I had is basically the same unit as the USASpec one from crutchfield. Neo has come out with some newer models you can check out at Mp3YourCar.com - Connect your iPod to your factory car stereo with one easy adapter connection kit