I can't think of another fishery that flies under the radar quite like the Mississippi River upstream of Minneapolis for smallmouth bass. Based both on my experience and all I've ever heard from others who have visited my favorite sections, the fishing is simply exceptional. I believe the main reason it gets minimal acclaim is its location. Minnesota is the Land of 10,000 Lakes, after all, and anglers there have a "lakes" mentality. So many lakes in all parts of the state offer great fishing, often with fishing resorts on their banks, that the rivers get overlooked. Closely related, Minnesota is walleye country, and many anglers there simply don't care about smallmouth bass.
I've always likened the character of the fishing to what you can experience on a large Appalachian or Ozarks stream at its absolute best, except instead of catching smallmouths that average 3/4 pound, you catch fish that are mostly between 2 and 4 pounds, and any given fish that bites could be an absolute trophy. We didn't have means to weigh my biggest from the river, but I've caught a lot of big Great Lakes smallmouths, and I'm certain a giant that I caught on a Zell Pop in the Brainerd section about 10 years ago was very close to 6 pounds.
Beyond producing fast numbers, the stretches of the Upper Mississippi that I like best offer a style of fishing that I think is really fun. It's classic river fishing -- staying on the move and targeting eddies, current run-outs, ambush points behind rocks, etc. The fish are mostly shallow and in predictable river spots, and if you pay attention you can really pattern them. They'll also crush topwater during the summer. During one three day-trip, I kept a popper on one rod and a shallow dropshot on the other the entire time and never considered trying anything else.
The Mississippi River has many great smallmouth fishing sections, including a few above dams that can be fished effectively from a bass boat. However, the specific sections that I'm mostly talking about are the Monticello/Elk River section, which I'm guessing is about 30 miles upstream of Minneapolis, and the Brainerd section, quite a bit farther up the river. Both of these sections have to be floated, fished from a jetboat or waded in select spots because of plentiful shoal habitat.
I'm not sure when I'll have opportunity to return to the Upper Mississippi, but I'm constantly looking forward to that next time!