This is my 3rd harness for my boy (XS, S, and now M) and is the only brand that he hasn't been able to shimmy out of. Every cent of this harness is worth it just for the peace of mind and I'll never go anywhere else for one.
Richard wears his harness on walks, but also for car trips, since he hates cat carriers and will scream himself hoarse, while in his carseat he just chills.
We live rural and he does a twice-monthly 2hr drive with ease and comfort thanks to this harness. It's so secure that (at slow speeds) I don't hesitate to put the window down so he can stick his head out.
The harness is comfortable enough for him to wear it for several hours when necessary, and he'll eat, sleep, jump, climb, and snuggle with it on.
I recommend this harness to everyone.