I have owned four .22 mag rifles. Three were Marlins and the current one is a Ruger 77/22 mag. The Marlins shot OK, but I have never had what I consider, great accuracy for the cartridge. The Ruger shoots better than any of the Marlins did.
As to usefulness, in my mind it's a light-weight cartridge for smaller animals at relatively close range. I have used it quite a bit for groundhogs and it works OK. I have taken one coyote that was head shot at 75 yards. It dropped like a hammer. Also, I took one small bobcat at about the same distance. It dropped on the spot, but was still alive when I approached. The bullet had exited near the spine and it was paralyzed.
I used Winchester 40 gr. hollow points for the groundhogs, coyote and bobcat. Probably the most accurate round I have fired was the Remington 33 gr. accutip. I have only used that one for crows.
Here's some examples of what the Ruger has done.
I really like the Ruger's rotary magazine.
Having said all this, I don't predator hunt per se with the .22 magnum. Both the bobcat and coyote were targets of opportunity
while I happened to have the .22 mag with me. I know it dropped these two with no problem, and I know a lot of guys use the
cartridge with good success. For me personally, I like a bit more cartridge if I am seriously predator hunting. I have a feeling
though that the .22 mag might be a good fox round considering fur-friendly, though I have yet to put that to the test. If a
rimfire was all I could use, then I would use the .22 mag and make it work, ie closer ranges and more precise shooting.
But, since I am not restricted and can use what I want, I reach for a cartridge that has a bit more to offer.
The cartridge is capable, taking range, bullet, and bullet placement into consideration.
Good luck.