Here's the Sonic from the Denny's Sonic Underground promotion. What's so Underground about him? See that tiny gray bit in the first photo? Its his magic guitar necklace that appears in the inset (bottom right) it's just a little gray scrap with black detail to represent the guitar that was shaped like his head, and shot out music lasers from the show. Why? Because music lasers. His tag is identical to Knuckles' (seen above) It's a decent enough likeness, especially for a promo doll. Do notice he only had 3 fingers though. The dolls weren't free, they were 2 dollars with a meal at Dennys. 

In the gallary is the Denny's Knuckles plush from page 1, but here you can see him better because he's out of the packaging. For a promo doll, he's pretty decent. The shoes only have 4 bolts instead of 6, the fists are kind of small...but that seems to be about it. The face and spikes look all right as well. Notice the "Sonic Underground" labeling on the tag there. Knuckles DID appear in the show, for only 2 episodes.

There's nothing "underground-ey" about the doll because Underground didn't actually have anything to do really with Sega or Sonic characters in any of the main plots. (The only thing Sonic about it was his likeness, most of the time) No doubt it caused a conflict with merchandising.