Seems I'm just reviewing booster seats. Anyway after the Britax one this is an awesome seat, because it's simply so versatile and convenient. We bought it for a holiday abroad and as an extra booster seat and it has been better than we hoped for. The travel pack is big enough for a small rucksack which we put inside loaded with things to play with on the plane. The booster seat is sturdy and comfortable and worked well as an extra seat for a second car and for the trip to the airport and in the hire car abroad. It was far better than the seats we got from the car hire firm last year. You can also use it on the plane. For example my daughter can see out of the window with her seatbelt on. This was ideal especially for the take off and landing as previously she was too low to see out of the window and kept trying to remove her seatbelt. Once at the airport we got a taxi which had no child booster chair. So it saved us time trying to find one from another taxi. By the time the cases were in the boot, we were securely fastened in. This was the same on our return back to the airport. The taxi driver on our return was very impressed with this seat asking where we'd got it from so that he could offer it to his clients in the future!!!
We have one of these for emergencies! It lives with the Grandparents and if they need to collect my eldest from school or transport him somewhere they can easily do so without having the fuss of a full size high back booster seat. My son likes being able to carry it himself to!
While I would always recommend a high-backed booster seat for safety reasons, we do own a Boostapak for travelling. As a car seat I found it very easy to fit as the instructions are printed onto the seat itself. The booster seat is a little more advanced than many other booster seats as it has a fold-out fabric flap which you sit up against the main seat of the car (note, the flap has ears which make it look a bit like a little monster sitting in the back of your car!). The flap has a red seat-belt guide that helps you place and locate the car's seat belt correctly to ensure maximum safety. The cover of the bag/seat is removable so that you can give it a good clean as and when needed. As a back-pack the Trunki Boostapak is quite heavy because of the firm seat shell inside it. As such, it may not be suitable for some small 4 year olds, even though they are officially old enough to travel in a booster seat.
A great alternative to carting around a full size carseat for the Wee Man, with the added bonus that now he gets his own seat on the plane, he can use this to be able to see out of the window!