Some good points brought up here in this thread that I didn't think about (cheap 9mm is about the same price as cheap .22 Mag is) and some I've already concluded.
In a single action revolver, a revolver fitted for the length of a .22 Mag cylinder means a spare .22 LR cylinder can be used, which is something Heritage provides with most of their Rough Riders, North American Arms in some of their revolvers, and Ruger with their convertible Single Six.
The .22 Mag gives you more power and good self defense JHP ammo, which is the sole reason to own an NAA mini revolver. In a full size revolver, if you were looking for a really good trail or trapping gun, a convertible .22 is hard to beat. If you were not well off financially, the convertible .22's like the Heritage give you a cheap gun that lets you practice with cheap .22 LR and use for home defense with .22 Mag; for women or young teens, these are not bad options.
That said, if you can swing the extra money and take the recoil, a 9mm revolver is better. More money? Yes, but better for defense, just as easy on the wallet compared to just shooting .22 Mag. The Kel Tec pistol is a cool idea, I just don't trust .22 Mag in an autoloader.
In rifles... I just don't see the point of .22 Mag vs .22 LR for distances under 100 yards, especially small game that you plan to eat, and for anything over 100 yards that's for varminting there are better options; .17 calibers, .22 Hornet, ..327 Federal, .30 Carbine to name a few. The only difference again is price, but unless you're living out West and are on a ranch and have to shoot that .22 Mag rifle of yours every day to keep coyotes or foxes out of the hen house, I think you can afford to get something better that's a bit more in price.
One thing I don't like with .22 Mag is that I notice a lot more FTF's with it vs .22 LR. Idk if it's the particular ammo I'm shooting or if the brass in the rim is thicker or if there's something wrong with all of my .22 Mag revolver cylinders, but there are times I can go 100 rds with .22 LR and get no failures, but do that with .22 Mag and it's a guarantee 5 of them are not going off. Thankfully in a revolver I just go to the next chamber, but in something like the PMR 30, especially if it was when I'm on the clock in a competition or in a defense situation, that's a critical failure.
I have three .22 LR/Mag revolvers, got them because I wanted to plink with the .22, but have the .22 Mag for more power when it may be needed. I have no rifles in .22 Mag, I just have no use for one. If you wanna get a .22 Mag, get it in a revolver, preferably a single action.
In 2019, there are some niche guns that .22 Mag just works in, but otherwise I think it's a cartridge that lives off it's association with .22 LR. Back when it was introduced in 1959, there was no hyper velocity .22 LR ammo, there was no .32 H&R Magnum, hollow point bullets weren't a known science, .17's weren't a thing, .22 Hornet did exist, but to shoot it in a revolver wasn't practical.
The .22 Mag was a great cartridge for its time, but a lot changes over the span of 60 years.