Like dazz, I used the Nissan unit ( mine was Nissan Connect) in my Juke when I had it as a company car. Worked well with iPod touch, memory stick, sound was spot on and very easy to navigate through tracks.
I have a BMW estate now (making up for being hung like a baby carrot) and of course the stereo is much more simple. Does have DAB though an one if those 3.5mm jacks, so all is not lost. If I were putting an aftermarket unit in, I would definitely make sure the unit worked with iPod/USB sticks, or at least one or the other. So much better than loads of CDs in the car.
Edit: one thing I would say is that if you are using an iPod, make sure the manufacturer allows you to use the normal iPod cable. BMW charge some crazy fee for their own one, apparently the one that came with the iPod does not work. Which is pants.