I had that one and I returned it. The first time I put Roxie in it she somehow managed to get over the side and was being choked. I almost got into an accident because I panicked.
I now have
Top Paw™ Booster Seat - Sale - Dog - PetSmart, which isn't fantastic, but it's better than the first one. I keep the legs folded under. I have to be honest though, I don't take her too many places other than the vet, my parent's house, and Petsmart.