We have a 70lb doberman. I keep the seats down and have no issues. I've tried with 1 or both seats up and it does not work for us but as it worked for the previous poster it all depends.
One other note. The back hatch window is a heat sink. We do a lot of long trips with the dog and it was a sauna back there. We ended up tinting the rear hatch and rear windows even though we had the factory sunshades.
IMHO, the x5 we had previiously worked much better as far as the dog is concerned. Good luck.
2012 550I xDrive GT
2010 Volkswagen Golf TDI
Gone but not forgotten:
2009 X5 35d
2007 335i European Delivery with Performance Center Re-Delivery * ED May 4, 2007 Link to ED Trip Report / PCD July 5th, 2007 Link to Performance Center Re-Delivery Trip Report
2007 X3 European Delivery with Performance Center Re-Delivery
2002 330ci, 1997 328i, 1990 325i