How bad is it? I think it's pretty good, with poor editing in the dubbed version.
Should you see it? Yes, but not because it's so-bad-it's-good.
After "This Island Earth," I think this is the best film that MST3K parodied. The original Italian version runs 15 minutes longer than the dubbed version and is superior. It's filmed in a widescreen format and has an epic feel. Reg Park plays Hercules and he fights an upright dragon and an army of identical soldiers before Atlantis (and queen Fay Spain) get sunk. Though it is campy, it's very enjoyable camp.