A long time ago in Las Vegas I saw one of these for rent...and I just HAD to try it out! Near as I can tell, it was the same model as you bought...quite different looking from the WW2 military version!
My impressions were that it was really heavy, even more so for an SMG. It sort of feels like it's 3 times bigger and heavier than an Mp-5. The compensator may have an effect, but the rake on the stock means it'll climb if you are just medium sized guy.
Two round bursts are easy to keep "on" at 25m, but the third is climbing, and after that it's either ceiling or sky. Even the stick mags are heavy with 30 rounds of .45acp...
I have not tried the drums, from what I've read though, if they are in good condition, they ARE reliable (unlike most drum systems that usually aren't worth a damn). I believe there are 50 round drums, and even a small number of 100 round drums. They MUST weigh more than a car when full of ammo, and I'm VERY certain that the action must be locked back to the rear before you load the drum onto the SMG or it simply won't seat properly. My guess is that the drums are fantastic fun for the range, and horrifyingly impractical for use in combat, which might explain why the US military never adopted the drum system!
I think yours is in nicer shape than the one I rented...not really surprising since mine was a heavily used rental, whereas yours is a collectors' item!