Having just had the pleasure of installing one of the New MINI Stuff AV upgrades into my R56, I can honestly say it is one of the best improvements you can give the car. Especially with the pretty rubbish standard kit that comes from the factory (Boost stereo and cheap speakers).
The kit I have is a pair of Kenwood 6x9 3-way units in the back panels, a pair of Alpine woofers in the lower front door and a 700W Kenwood 4-channel amp. All wiring came with the kit. I also bought a set of Vibe 10cm speakers for the mid-door positions (Halfords).
There is frankly a frightening amount of trim removal to be done to get to the speakers and wiring, so I recommend getting some trim removal tools (lots on eBay). Take your time though and you will get it done.
I would say it took me around 12hours in total over several days, but it was a voyage of discovery, with lots of careful planning and some repeat steps as I worked out how to do it.
The sound is now excellent with plenty of volume and great clarity for lots of different types of music. It needs. A little bit of balance tweaking but overall I am delighted.
So should you fit one yourself?
You can get great improvements with the Channel Swap kit and leaving the rest alone, or upgrade the fronts by themselves to something better. Much less dismantling and cost. I don't believe it will sound quite as good nor go as loud and keep the clarity but it is a fairly quick and easy job.
Being in Ireland it was not practical for me to have Mike from NMS fit a kit, but seeing how much time is involved I think their pricing is great value (if you are over there to have it done). Glad to have done it myself bough as I learned more about what my car looks like naked
Any questions give me a buzz while everything is still fresh in my mind!