Those are mainly a substitute for a "real" TSMG for those who want the Thompson image but can't affor the real thing. They are heavy for the caliber, and with the ten round magazines (even the drum is now 10 rounds), they are not nearly as much fun.
And you can't use regular TSMG magazines without reworking. This may be acceptable for a 20 rounder, but no one in his right mind would destroy an original drum (costing twice as much as the gun) to make it work.
The other problem is that without the advance primer ignition of the open bolt gun, it takes two men and a small boy to cock the gun.
All that being said, they are pretty reliable and still fun.
Jim