I have a bubblebum and use it frequently; it does have the 4 years/40lbs restriction (child must meet both requirements). But it's a great product for trips where your only car trip will be the cab to/from the airport. It's also worthwhile to keep around as an extra seat if you have a playdate. It packs up very small and fits in a medium sized purse. We use it around town if for example we are going out in the evening and will want to grab a cab home.

The bubblebum is essentially a booster seat though -- there's no reason you'd use something like that on board a plane. If you want a safety harness for the plane, you should get a CARES. You can't use that in a car, though.