I've got the Pioneer 4200DVD, which is the double din unit immediately below the Sat Nav units in the Pioneer range. It's just been replaced by the 4300 DVD which costs about £400 and is basically the same thing. I also got the additional Bluetooth Unit (CD220) which is about £120 as there is no Bluetooth built in. Still much cheaper than the Sat Nav Units.
Sound quality is excellent and there are loads of features including full ipod control, album art, MOSFET amp 4 x 50Watt, 3 high volt pre outs, sonic retrieval (restores the lost harmonics to your mp3s), 8 band graphic equaliser, 3 user-defined GE settings.
If you're not bothered about Sat Nav I would recommend this solution (and to be honest I think the stereo in these cars is a bit low down in the centre console to use safely as a sat nav).
Looks great in the Brera/Spider/159.
Bear in mind that with any replacement head unit you need to budget £50 for random bits like Fascia for the car, aerial adaptor and ipod cable. If you're getting the car audio place to do the fitting, budget an extra £100. It'll take them a few hours so you get your money's worth...
As a bonus, it's fronted by a lovely 7" touch screen and it plays DVDs, CDs and SD cards too. However the media it plays are not the main reason to get it; it's got very good sound quality and is the cheapest Panasonic unit to route the ipod cable round the back instead of in the front so you don't have an ipod and cables loose in the cabin.
Pioneer AVH-P4300DVD - www.pioneer.co.uk