I have spent many many days on the Rush. I also did a couple of days worth of survey work with the WI DNR. It is chock full of fish!
The Rush is a great trout stream. Mostly browns, some brookies in the cooler areas, and an occasional rainbow from stockings, but I think that the days of rainbow stockings are behind us. Water quality is improving (good) and temperatures are falling (also good) but those things tend to favor smaller, more abundant trout over the sharks that were present 10-15 years ago.
The closer you get to Pepin, the more cool water species begin to show up. Smallies, Pike, and even walleyes can be found, although not in great numbers. Yes, I have caught all of these species in the Rush on the fly!
If you want any more info, drop me a PM.