Fun toy? Absolutely, yes!
Investment? Absolutely not.
If and when the legalities of the NFA ever get challenged you can bet your ass that every idiot with more money than sense will be fighting tooth and nail to keep the laws the way they are. If the NFA gets overturned and/or the registry is open for new guns their multi-thousand dollar 'investments' become nearly worthless in the stroke of a pen. You can bet people with 10-250k or more wrapped up in this sort of thing will do nearly anything to keep that from happening.
Will there be exceptions to that rule? Of course but make no mistake it will be the rule.
Ek
While i do agree with you that there will be some MG owners who would not want to loose the money they paid for their guns, I actually believe they would be the minority. I know at least a dozen MG owners (myself included) and I do not think any of us would give up the chance to buy new, cheap MG's just to protect what we have invested in our current ones. YMMV