I had been lusting after a friend's BA Sleeping Giant for a few years and FINALLY pulled the trigger on one of my own. I have an extra wide Therm-a-rest that I already use for car camping (it's an older model, I think it's called the CampLight?) that is 25inches wide. So I got the wide/long version of the Sleeping Giant and it's a perfect fit. I have never slept so well in a tent!
I have also used my ultralight/backpacking POE sleeping pad in there for times when we are on our way to a backpacking trip, but still have one or two nights car camping before we get to the start of our intended hike/backpack. And then leave the Sleeping Giant in car while we are off in the woods/desert.
Sure it takes up space, but if you have room in the car, it makes your sleep much more sound! I also find that the memory foam WILL squish down pretty small, so if you have a little bit of extra space in a duffel, you can cram it in there and then it really doesn't take as much extra space as you'd think.
I have also compared this to the Therm-a-rest dreamtime and think BA's quality is MUCH better, and they paid a lot more attention to detail.