Our bullmastiff is 55kg, she is quite a large dog. She has enjoyed the additional space to stretch out on long drives with the supported flat base which allows about an extra foot of support off the edge of the seats.
The vented panel at the front is also very handy to allow air flow through to back, and so she can keep an eye on where we are.
Due to the high quality and design of this product with additional reinforcing & also the flat base design, (unlike other cheaper Hammock style alternatives), the straps that secure around the headrests are very sturdy & I don't see them giving out anytime soon!