The only reason I'm not out trying to get a class III right now to get hooked up with an automatic weapon right now is because there's so much darn work involved that I don't feel like dealing with to get a weapon that I don't need. I've met class III guys with full autos that have allowed me to fire their weapons before. Hell, you can go to Scottsdale Gun Club here in Arizona and pay about 30 Dollars to rent a full auto HK MP5. Is it the end all be all? Nah. In my experience, full auto is fun, but it isn't always practical. If I were going to buy one, it'd probably be the MP5 I mentioned. I like it, and I would only need an auto for room clearing. It just doesn't land enough rounds on target in full auto mode at longer distances for me, nor does any AR-15 or AK I've fired. I'd rather just fire a controlled pair semi auto. For me, choosing between a full auto weapon of any kind or a nice new bolt action or 1911 will always land me with one of the latter. Would I buy a select fire weapon if it were easier? I'm sure I would. But probably just one, since that's all I'd really need to get the occasional full auto hunger out of my system.
As for their legality, they should be 100% legal for citizens with no mental deficiency and a good knowledge of gun safety. After all, how many gun crimes have you seen commited with a long arm of any kind compared to handgun crimes?