After pricing replacement summer RFTs for the M Sport tires I decided it just isn't worth it to replace the Pirelli P7s when they wear out. I used our space saver spare from the 128i to determine how much usable space the spare tire is going to take up. The X1 spare has a higher profile sidewall so it will have a slightly wider diameter (keep that in mind with the photos).
With the tire flat on the carpet, I would say usable space is not affected much for normal stuff such as going to the grocery store or Xmas shopping. If the smaller rear shelf is removed, the tire can be strapped to the back of the seat (this is our setup in the 128i). Large items that require the seats to be folded down would require you to remove the spare (which is what we had to do in the 128i prior to the X1).
Add a stroller to the mix and things get a bit more cramped. Stroller in the picture is the MacLaren Techno XT stroller. MacLaren also makes a BMW version of their Qwest stroller. We will be getting the BMW stroller bag for our Techno XT (http://www.shopmaclarenbaby.com/BMW-...efault,pd.html
). I prefer the tire flat on the carpet based on my testing.
Trips requiring luggage and a stroller as large as the MacLaren are going to be cramped even without the spare and may require use of the back seat to store some items (I don't think its safe to fold down part of the back seat for items with a baby in the carseat).
I hope these pictures are useful to those considering ditching the runflats and carrying a donut spare (which we have never regretted in our 128i and E90 335i).