Airbags are easily misunderstood. I've never installed or used them, but I'm getting ready to soon. I've done alot of research on them.
Airbags CAN be used like hydraulics to make your vehicle "hop." Its just not like hydraulics in how it acts. Hydraulics are capable of intense extension speed, making them suitable to the high-flying jumping noses you see in rap videos. With extensive modification (and even more money) air can be made to rapidly inflate and deflate. It takes huge air tanks, 1" ports and hoses, and huge solenoids. You won't be yanking the tires off the ground, but it will look like hydraulics othewise.
Hydraulics use a liquid which can't be compressed, therefore their height is dependent on how much volume of fluid is in the cylinder. Since the weight of the vehicle is constant, the PSIs remain the same. Air uses a compressible gas and its height is dependent on PSI differences. It also stores the spring air in a flexible rubber bellows making the PSI wildly affectable. Air springs are rated at a max PSI, weight rating, and installed height. If you want to make your truck air ride, set it at the desired height and measure how tall your coils are (or how far it is from the axle tube up to where you'll mount a frame mount). Let's take a hypothetical example of an S-10. You may want springs with an installed height of 7" for the clearance you desire. Since you want to be able to haul some gravel from time to time, maybe 1800-lb springs is what you want. Once they're installed, add air until they come up to 7" and note the PSI. Ride height is where you get optimum RIDE and road characteristics. PSI is inconsequential. It may take 75 PSI in your truck to ride at 7", but the same springs in a plymouth wagon may take 120. The big benefit to air is that when you ride around town with nothing in the bed, 75 is optimal. Throw in a scoop of gravel and pump it up to 7" again while the load is on. Buy a cap? Your new benchmark may be 80 PSI. Its flexible, but one thing it can't do very well is make it a multiple-height vehicle. Straying above or below the ride height causes exponential degradation of the ride quality. At lower heights, it gets "see-saw" like front to back, and allows for unneccessary compression and bottoming out. At high levels it gets really bouncy allowing too little movement. If you're willing to put up with excessive tire wear, bad ride, and other issues to get a multiple-height look, its a great way to go. At least with air, when you return to ride-height it rides well.
Pros:
-adjustability for multiple loads
-cadillac-like ride
-low-maintenance
-Unlike steel springs, by design they work really well at all weights (within the limits of your GVW). Steel springs are a trade off of weight handling and passenger comfort. My F250 with heavy duty springs will haul tons of weight, but absolutely punishes you on anything but brand new pavement. My 66 Bonneville rides like a marshmallow, but one cooler of beer in the trunk and its draggin the tail. Air would solve the problems on both of these vehicles. On the truck it would maintain the weight handling and increase the ride quality. On the car it would improve the weight handling (not GVW) and maintain ride quality.
Cons:
-can be expensive
-corners like a cadillac
I think too many people get discouraged with air because they expect the low-rider on 20" wheels attitude and they end up just wearing out tires and denting oil pans.
Use it for what its intended and it won't let you down from what I understand. If you are looking for good ride, superior load control, and don't mind a step down in handling, air is great. If you want handling and carving corners, stick with steel.
Good luck
Curtis