Great harness. It's the cheapest option for keeping the dogs safe on the back seat and to prevent dogs from turning into ballistic missiles during a crash (likely to seriously injure both you and the dog). The only problem we have experienced is that the adjuster on one of the harnesses doesn't stay tight when adjusted, which means our dog has managed to back out of it once or twice. Be sure to check the adjusters aren't too loose, as soon as you get the harness. We have two dogs on the back seat with this and it means they can adjust their positions, without being a danger.
Good harness but we bought the small size to fit the chest size of our Lhasa apso and found that the neck hole was too tight and the chest bar too short. The medium size that we bought for our other dog(PugXLhasa) fitted better.
This is a good harness but I would be wary of using anything in the seat belt socket other than a seat belt! My dog's Clix harness recently jammed in the socket (after several months' satisfactory use) and it is costing me £97.50 to replace the socket, as using the Clix in it is has voided the warranty.