Another long-term product review for ya. I've been a dedicated Powertank user for over 10-years. There's a reason.
In my opinion, Powertank simply offers the best portable onboard air option out there - period. The regulators don't ice up and slow down with extended use like some cheaper units may. And Advanced Air Systems, the company that makes Powertank, is continually innovating and coming up with cool ideas, from different tank sizes, regulator and mounting options, hoses, accessories, you name it.
At the moment I've got four Powertanks. Two 10-lb bottles and two 5-lb bottles. The 5-lb is a perfect fit in my tight flattie. I have it mounted horizontally on the rear fenderwell inside a sweet, lightweight aluminum bracket by Advanced Air Systems. Although no CO2 tank works when horizontal, the bracket offers easy-access so I can pull out the Powertank to use it. It gives me enough juice to air up several tires and the regulator is teeeny and the other 5-lb bottle is small enough to toss in the tow rig as a backup, although I've never run through the full 5-lbs with normal trail use.
The two 10-lb bottles come along in my larger vehicles. I've got a rollbar-mounted bracket in my M-715 behind the rear seats that takes one 10-lb bottle. I'll usually toss the other in the tow rig or in the back of the YJ if I need to air up for a long drive after a trail ride.
Although their limited CO2 supply still doesn't make them a replacement for a dedicated onboard air compressor if extended air tool use or airing up a fleet of tires is in your playbook, in the decade-plus that I've been using Powertanks as a hardcore off-roader, I've never been left wanting. There's an amazing amount of energy inside even the 10-lb bottles and thanks to the company's stellar regulator systems which have never failed on me, you'll never be unable to access it.