I have a 2012 X5 with 3rd row and can tell you that the 3rd row is best for older children and toddlers in a booster seat. It's pretty cramped space. I have two boys ages 2 & 3, and their car seats do not fit back there. The 2nd row bench is 60/40 split, so when we travel with another adult I strap the boys cars seats to the 60 bench, leaving the 40 bench for an adult and if need be that seat back pulls forward to accomodate access to the 3rd row.

With the 3rd row up BMW has a special cargo liner just for that space, but you may look into Weather Tech for a tri-fold liner to use when the 3rd row is flat.

Joe