You had it right...."I've always wanted a Tommy gun, but...."
If you're looking for a civilian-legal, historical firearm to collect and shoot, the Thompson's will fill the bill. If you're looking for a defensive arm, there are lighter, more modern guns out there.
Since most of would like to have a full auto TSMG, but either can't afford one, or live where they are a no-no, the semi version will do just fine.
The Thompson's are a HOOT to shoot, and are reliable, and accurate.