I camped on the Willow Flowage six times from 2002 to 2004.  It can be nearly deserted, and you can have trouble finding an open campsite.  In July 2003 I had to borrow the corner of someone else's site because I couldn't find one (and it was raining).  I stopped going because on my last two trips (July and Sept. 2004) it took me a long time to find a site.

If you go, be sure to check the WVIC site.
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The lowest safe water level is 7 feet below full pool.  Lower than that and a lot of stumps will be just below the water, which is so dark you can't see them until you're nearly on top of them.

If you want a more wilderness-y experience, try the Turtle-Flambeau Flowage.  It has a lot more campsites, and the eastern part of it is motorless.