I do not own the Super Yelper, but I do own a few scratch boxes...  My favorite is from Lonnie Gilbert.
They are my go-to call for a long sit...  Subtle purrs and clucks, with a soft yelp mixed in, and every 20 minutes or so, I get on it.  
Currently, it is also the most fun call to play for me...
Last year I had a bird that I could barely hear gobble, and was surprised how he would gobble at those quiet clucks and purrs.  Made me realize just how good their hearing is.
Couple years back, I was sitting against a fallen tree...  I took a break to eat lunch for a bit while doing some subtle calling to a bird some distance off...  After I ate, I decided to get on the call a bit and see if I could locate the bird.  Turns out he was on the other side of the fallen tree I was sitting against.  I about jumped out of my pants from being startled by that gobble...  The remnants of my lunch when flying, as I quickly raised my gun and that bird came running around the other side of that tree right into a load of Hevi-shot....