I've used an Air II since it was first introduced. I still use one today--second generation.
It works just as advertized. With use, you "learn" exactly where it is--I can play those buttons like a flute if I want. The response is totally automatic. It doesn't drag in the sand and tends to stay clean.
It HAS SAVED MY LIFE ON 3 OCCASIONS! I was nearly knocked out in 70 ft of water by the butt of a speargun when it recoiled as the spear hit a rock under a sand bottom. The impact "rang my chimes". My primary was tossed behind me and snagged. No amout of arm sweep cold retrieve it. By instinct, and familiarity of use my left hand found the spare regulator and I was able to breathe. That is the precise reason Scubapro touts this product.
The other two occasions were other divers out of air and they took my primary--naturally, they wouldn't give it back! No panic, no problem,the left hand "knew" where the air was and it was totally automatic. Instead of two dead divers, there were two live divers.
Yes, the device has its drawbacks. It's too expensive (Scubapro-naturally). But this expense is probably LESS than a pony bottle with an attached valve and regulator and attachment devices from the pony to you main tank. You have to resort to your back up regulator and give your primary to a diver in trouble--cumbersome. The hose is stiff because of the hose connection. BUT, there is air, gobs of air--in most cases. No way are you going to make a slow, safe ascent and safety stop with a Spare Air. It is completely possible with the AIR II.
Is a second, pony bottle with a separate reg a better choice for a back up--probably, but it is cumbersome and you have extra hoses--you start to look like hoses into the Michelin Man! Is it appropriate for deep diving or tec diving--probably not--it's not really redundent--neither is an octopus rig from your main regulator. So you're back to the volume of a medium pony bottle and a second regulator.
Does the AIR II live up to its promise...you bet...I'm living proof of the concept...the developement of an engram (a learned, engrained neural pathway for a particular muscular action) which allows me to find the regulator time and time again, blindfolded, because you have to learn to find the buttons for inflation and deflation...and it's not rocket science to simply bring it to your mouth.
So, despite any drawbacks, I'd buy one again. I've never had one fail--and it gets more punishment than my primary because fresh/salt water is routed through the same mouthpiece you are supposed to breathe through. It gets serviced regularly and it works.
Larry Stein