Sometimes necessity really is the source of invention. I think I may have waited so long with airguns simply because finding the time to shoot the "real" ones was never a problem before now (kids, different environment, etc.)...it really was a classic epiphany moment when I first thought, "why not?"
Cost is certainly a plus point for airguns. As of this writing, a tin of 500 (.177) pellets--and quality ones, at that--goes for well under $10; where I'm at even .22 rimfire brick ammo is at least $20 per 500; if you want good stuff it approaches $10 per 100. Centerfires, of course, start almost an order of magnitude above even that, and continue.
Noise is another selling point. Not everyone lives in a place where it's realistic to take two steps out your front door and start hitting steel with a firearm, but airguns give me lots of options.
And I'm intending to try out Airsoft as a "serious" training aid, as well. This breakbarrel springer pellet rifle will be excellent for marksmanship fundamentals, and some basic gunhandling skills (this type of action is so much handier than the tedious multi-pump varieties), but if I'm understanding things correctly, Airsoft can get you authentically there, if you're careful when shopping.
If you can keep your gorge down when watching*, consider this video review of a 1911-clone Airsoft gun. For the whole review, look here:
To go directly to the shooting part where you can see the piece in action, either scroll to about 4:26 or click here to watch it on YouTube. Note that the magazine and pistol should fit any standard leather, and between the low relative cost of Airsoft BBs and the several different gas-charging options available, this may well become a great "everyday" solution between dry-fire and live-fire exercises. I could get up and running with a piece that is functionally identical to the one I carry most of the time, with two or three magazines, gas and a supply of pellets, for under $300...and feeding it thereafter should be cheap. Some (but not all) of the Airsoft guns do appear to be just like the real thing, with the gas not only powering the pellet, but also cycling the action.
And the more I think about it, the more convinced I am that there is an important middle ground between dry-fire and live-fire, that this idea should cover well. For example: training in your own house with live fire is certainly going to be messy, and dry-fire is limiting because you can't realistically engage multiple targets. But the Airsoft option would allow you to do that, either without or even with the plastic BBs, if you have suitable backstops. Others will point out the actual force-on-force options that Airsoft makes possible as well, to train in tactics beyond the square range. It's intriguing.
Thus far, I believe I've found such an Airsoft trainer for both the 1911 pistol and the M14 rifle (other possibles like boltguns, leverguns, revolvers and the Kahr auto haven't been as fully vetted yet), and when I get around to each I'll be sure to post findings here.
* Explanation for that carefully considered statement is at the bottom of this post.