My favorite LGS had one on the racks the other day and I was surprised by how heavy it was when they let me hold it, they said it is as heavy as the original. I was trying to think of a reason why I would want one and the answer I found scared me.
The only good reason for me to own one is because I would go out and buy a real one and want to use the .22LR for training/practice.
The gun shop even has the 8mm kurz rounds for the original STG, pretty fascinating cartridge to hold.
So, I'm avoiding the .22LR version for fear it will lead to thousands of dollars later getting a "real one."