This seat is so great! Got the largest size, so it's roomie for my 15 lb Frenchie, but she'll grow into it. Works great either flat on the seat or as a booster. I think the other person didn't read the instructions, that's why they didn't know how the booster part works (you just put the strap around the headrest). My dog was secure, could not get out, but could put her paws up and look out the window. She whined for the first few min she was in there, but settled down quickly.
One complaint, when using as a booster, if my dog sits all the way to the back (closest to the backrest of the car seat), the whole thing starts tilting backwards. Not really badly (I had to adjust it after an hour), but I think that's a relatively minor design flaw. I could adjust it with one hand without looking.