I was looking into the GC mainly due to the larger size of the back seat. I have 3 small children, two in isofix seats and one just onto a booster seat.
Took out a 640 for a test drive with the family on board. Back seat space was actually not a problem size wise, both isofix seats were attached to the isofix points and the booster seat did fit in the middle. The problem was the console in the middle does protrude into the cabin and even though my little girl is only 3, she had to rest her feet on it. After a while she wasn't very comfortable and voiced her opinion as only a 3 year old can!
Boot space was ok, we have a Cameleon which went in without trouble but there was not much useable space left. The boot opening wasn't too good for getting the pushchair in and out but it was workable.
As much as I wanted it to work and tried to convince myself it did work Mrs B gave me the look that said enough to convince me otherwise