I would rate Mill Creek average in the big picture. Sure, the habitat improvement will help, but this coutnry's new found fascination with ethanol will definitely impace our watersheds neagtively. All the H.I. in the world isn't going to save streams like Mill Creek in the big picture. Sedimentation and pollution will be the bigger issue. As you said yourself, there isn't a big population of fish in Mill; the DNR's supplemental stocking of that stream definitely makes up for the lack of natural production. Will there ever be a significant fish population in Mill? Maybe, but it will take a combination of land use changes, continued supplementl stocking, and perhaps some special regulations to get it off the ground.
Best of luck,
D.A.