Yes, they ride smooth - but any spring should also with the shocks being a primary factor as mentioned. Both are relatively frictionless. To be fair, I would say they act like progressive-rate springs, and have no advantages over that simple system from that standpoint. However, they are adjustable for pressure and therefore ride height, allowing compensation for differing loads to maintain best handling and tracking. Again, to be fair, this has been handled by other means (such as rear air shocks and coil-overs) for decades, as the front has relatively little change in normal use.
So, I would say the pros to air bags are the 'fudge factor' in that the perfect length spring for ride height is not a concern. Air it up to your perfect height and go. Conversely, the rate cannot be chosen with most air bags, so if you need a stiffer rate, you can't just swap the next higher or lower-rate spring. In other words, if you have too much 'squish', it's an expensive change to a larger bag, as airing-up will stiffen it, but also raise it from the correct right height. One common partial fix for this is to use adjustable coil-over shocks in increase or decrease spring rate, as well as damping rate, just as non-bag systems have used. Adjustable-rate (progressive-rate) valved bags are available, but you don't want to know the cost.
Keep in-mind, though bags have been used in certain racing applications; most users of bags are not looking for ride quality and handling - they are after height adjustment on-the-fly. If looking for info on them, keep in-mind who wrote it. Someone looking to donk their car and slam it while waving at the cute high-school girls, or someone using it to adjust ride, geometry, and performance. Two very different purposes and uses with completely different setups and considerations.
My butt has a lot of miles on both, and I believe which one to use - and how it's done - should be based on the end-purpose. My 2¢.
David