I live locally and the campground is nice.  We've had a lot of rain this year and the local park that borders them (Shimek) has been closed all season except for two weeks.  I've heard the River Valley trails are muddy and I would think they'd have to be with all this rain and Shimek being closed.  The more the trails are traveled when muddy, they worse they get and they never close their trails. 

It's definately a place to put on you're to go to list for camping and riding sometime but I wouldn't advise it now unless you don't mind riding in the mud or roads.  Shimek offers a lot of nice riding so as a rule if you're traveling a distance, I wouldn't come if Shimek is closed unless you're coming for one of their weekend events and not planning trail riding.  Check out their calendar because they always have something going on and sometimes dances too.  It's a good idea to put in a reservation if you want electrical hookups because during event weekends, they book up.  Shimek does offer two primitve campgrounds with a hitching post and fire ring at every site if you can't get in to their campground.