Its the perfect multi-day river trip.  I think you should do it.  There is some class IV whitewater.. but you'll most likely stop to look at the big ones anyway (ladle, double drop, wolf creek, maybe some others) and can decide whether or not you want to run them. 

If it gets above 4.5 or so, I would consider just hopping on the rafts. 

I've been invited on a trip on June 4.... but it's looking like it may be way too high.