Over the years, I've seen several boats brought to our area (mid-Atlantic) from the Great Lakes. They've always been in outstanding shape.
Obviously, there's a selection bias here. Someone who would go to all that trouble would go to the GL and find the best boat they possibly could, right? I'm sure there are plenty of neglected boats up there. But what I've seen for sale, and what's been brought down here, are low-hour, shed-kept, indoor winter storage boats that are beautiful. Some of them were good-size, too- 50 and 54 Hatteras convertibles, just for one example.
Clearly the process of getting such a boat here is more complex than bringing it up the waterway in terms of when to travel, etc. But in my limited experience of these boats, the ones from the Great Lakes area were really beautiful- no corrosion intact pain and exterior wood, pristine engine rooms- just what you'd expect from a boat which spent five months a year in fresh water and the rest of the time sleeping indoors.