I've got a large cat and he'll fit in here sitting up for an emergency trip to the vet or a quick train trip - we don't have a vehicle. He is too big to be comfy to lay down though. I was expecting that with the listed dimensions, so 5-stars still! 
A smaller cat, a few kittens or chihuahua toy-size type dog would be super comfy in it all day though. The door design is kinda weird - but if you lay the carrier down on the floor, your fury pal can crawl right in after their treats. ;) I love how long the handle is and that it has backpack arms and even places to store a few cans of food and treats for an over-nighter, too. Very neat product, all around.