Ever since we got my dog as a puppy, he has hated riding in the car. He gets carsick and would drool, sometimes even vomit, while riding in the car. While researching treatment for dog carsickness, I came across some comments from people who said a booster seat helped with their dog's carsickness. It made sense to me, since I know with car sickness in people, it helps if they can look forward and see the road in front of them while driving.
I was torn between getting the medium or large size. My dog meets the weight recommendations for the medium, but other people said they opted for a larger size so their dog could comfortably lie down. I ended up choosing the medium because the large would have taken up half the back seat of my SUV. Glad I went with the medium - it's the PERFECT size for my dog (as a reference, he's about 21 pounds and measures 20 inches from the base of his neck to his tail). It's large enough for him to comfortably lie down and the sides are low enough to rest his chin on it. We installed it in the middle of the back seat and there's still enough for a person to sit on either side of him.
I did have a little trouble with the installation at first. I followed the directions, but it seemed a little wobbly and not very secure. I realized I could get a tighter installation if I pulled the seatbelt ALL the way out. While holding the fully extended seatbelt, insert it into the clasp, pull the lap belt part REALLY tight through the slots in the back of the booster seat, and hold it tight while I let the shoulder belt part fully retract. By doing it this way, the seat belt locks and you get a really tight installation - the booster seat barely moves.
I do plan on buying a different strap to connect my dog's car harness to the seatbelt, though. I read several reviews from people who said their dog somehow managed to step on the latch to open it slightly then got their leg caught in the clasp. Also, the fake sheepskin sheds a little, but I'll probably just put a small blanket over it, so not really a big deal.
I took my dog for a ride today to test it out. He jumped right in it when I told him to go to his "bed". He layed down during most of the ride, but was still able to see out the front window. He would even sit up and look around when the car was stopped at lights. He still drooled a tiny bit, but not nearly as much as usual. I'm hoping he'll grow to enjoy riding in the car once he gets used to his new seat. If nothing else, at least it's a much more comfortable and safer way for him to ride.