The reservoir itself is kind of meh unless you like fishing or staring at water. But the drive up is beautiful, particularly if you go when the sun is low in the sky. And when the aspen are yellow.
The narrow, twisty road starts in Virgin at about 3,500 feet and climbs to 8,100 at the reservoir. Along the way you pass through sort of the back side of Zion National Park.
Eventually, you get as high as the top of the peaks of Zion.