It's a long hike and you'll be crossing a lot of lava fields. It's nice, but I wouldn't do it again especially when it's raining and visibility is bad.
The first 5 miles or so is on a lava field, there was some venting when we went. Then you have a couple miles in the forest along the rim of a crater, and then in and out of a couple more lava fields before Napau. Not a bad hike, minimal elevation change, kinda long. If you have never hike long distances on lava before you may want to do a shorter hike before committing to this one.