Very Easy to fit! well, I think...I had it professionally installed. Sounds fantastic. I was advised that installing one where the cut out for the Audi one fits would be more costly/hassle (rear parcel shelf would need cutting - no pre cut vents, separate amp needed and wiring etc) and may not have the same sound as the enclosed sub. It would also hang down into the boot space - (the audi Bose one is really thin and from what I've read, pretty crap too). I opted for the small active 10inch VIBE after hearing the quality from such a small unit. I've also got VIBE fast plug connectors to remove it easily when the full boot space is needed.
If anyone upgrades their headunit to an aftermarket unit which works with the bluetooth/steering wheel controls I would be very interested to hear from them!
Cheers