Did the Death Hollow hike last August (except we started from Hells Backbone, and hiked downriver to Hwy 12.)
A very nice hike, with water all the way, so the heat isn't oppressive. Plus, some of the nicest swimming holes you'll ever find. Lots of springs pouring into the Death Hollow, some really neat. The spring at the beginning of perennial water is a huge clear pool, and from there on down water literally pours out of the rock in a number of places. A real southern Utah classic, without all the traffic you'll find in other more popular areas.
One qualification--if you start from Hells Backbone, there is a very long, hot hike till you hit perennial water. Unless you're after canyoneering, this upper section is easily avoided by hiking in at the Boulder Mail Trail.
If go in from Hells Backbone, bring a small inflatable pool float and a short section of rope or webbing (25ft?). There are some long deep sections of narrows, and having the option of floating your pack is welcome. Also, a few pour-overs require the rope.