I have two. One on .223 and one in 9mm.
The one in 9mm is actually practical to use. It is a little heavy but not that bad. I sometimes shoot in a place where Whiskey Tangos dump trash like appliances. I walk around with my 9mm SBR and the Beta mag and shoot up junk. I aim every shot but you can run though 100 rounds in a surprisingly short period of time.
The .223 one was what I bought at the end of the last assult weapons ban. There wasn't really anything I wanted that I didn't already have that was no longer banned but I wanted to buy something that the libs didn't want me to have, so I bought a beta mag. I only used it once or twice. It worked fine. No problems at all. I suppose it has it's uses. I wouldn't automaticaly say it isn't practical. But, for most purposes you would use an AR15 for, it isn't practical because of it's weight.
I might find an excuse to buy another one before they are banned again. Don't wait to the last minute to start buying stuff like this. They WILL be banned again.