Even though their layouts may not be "inspiring" to some, they were one of the first models/layouts (I hear they are MUCH better than AIR at Alton Towers). I would also like to say that compared to the Zamperla Volaire models (Tomb Raider/Time Warp at Wonderland), they are an amazing coaster.
The pretzel loop is an amazingly forceful element and the rest of the ride is very graceful. Some enthusiasts may find the Superman models "boring" however the GP LOVES them.
And with the locations of the 3 Superman clones, I wouldn't say they are over-produced. They're well spread-out in very different markets. It's not like boomerang coasters that are all within a couple hour drive of each other (in many circumstances).
I have a feeling we'll see a new B&M Wing Coaster before a Flyer at Canada's Wonderland