A balloon large enough to be effective in supporting a person would require a huge cannister.

I don't see why this is better than a parachute?

I think a balloon that you could jump into the air from the ground would be fun (but dangerous), but trying to deploy one is freefall sounds risky. you'll still need all of the harness and ropes that a parachute has, but you'll have pressure regulators and compressed gas, besides. No holes please.

Finally, on the pressure-suit part, if you have a problem at high altitude, you're probably better off to stay with the plane until you get to breathable air, then bail-out--unless it's on fire or something. But if you did get out at FL40, you may pass-out and be cold for two minutes, but you already said that you have automatic activation, with the chute or the balloon, you'll probably live through the touchdown, even if passed-out.

In the Steve Martin movie "A Simple Twist of Fate" he 'flies' in a surplus weather balloon. It's really cool and memorable.

It really happened. See http://www.snopes.com/travel/airline/walters.asp

With a Canister balon chute strapped on your back you may landin an awkward position, break your leg, it could save your lifeinspite of this this. The only question is the weight of thecanister and the amount of compressed air required to slow yourfall to the minimum. Light gases may do the job, but may be inflammable.