I have a 2004 Sienna with Firestone Coil-Rite air bags added inside the rear coil springs. For my use, it's almost all "pros", with the only "con" being leakage from the fittings if you don't get the tubing inserted into them just right.
Generally, the idea of the air bags is to allow the suspension to ride at the same height with a heavy load as it would normally with a lighter load. The rear is supposed to come down somewhat with load (almost every truck or van sits rear-end-high when empty), and the extra spring force of the air bags reduces the amount of drop. I load up my Sienna with our cargo (which drops the rear suspension mildly), hook up the trailer (with about 300 lb of tongue weight), and pump up the air bags to restore approximately the height I was at with cargo but before trailer. It sits level, and that takes about 20 PSI of air.
The fittings for most add-in bags are regular tire valves. I mounted mine (separate for each side) together on a bracket which is strapped to my hitch frame.
Please keep in mind that additional springs do not increase the load capacity of the vehicle or the axle
, but I find that they do really help, both to maintain appropriate ride height and to add stiffness for better control.
I suggest that Sienna owners also check out SiennaClub.org
for everything you ever wanted to know about Siennas... and more. There's really nothing unusual about the vehicle, but the reality of the internet today is that there's a forum for just about anything.
One last bit of Sienna trivia: in a comparison test, someone I know found that the Sienna rear dropped less under the same test trailer load than its two major competitors, but owners complained more. It seems that the curved roof line makes it look
like it's drooping more. I know that dragging the rear is a real problem (I dragged the hitch leaving a trailer dealer for a test pull with a friend's Sienna), but it is for all vehicles of this type, which is why air suspension is such a good idea.