I live in Hilo and travel to Volcano village and over Saddle Road monthly. There's a couple of mistakes in this listing that are misleading.
First, the name of the trail is Pu'u O'o Trail. There is also a Pu'u O'o vent at which lava is erupting but they are not the same.
Second, Pu'u O'o trailhead at "Saddle Road, near mile marker 22, Hilo, Island of Hawaii, HI" is about 20 miles from the vent and does not go to the vent. It is in the center of island at about 6,000' elevation, is free and on public land. It goes 6-7 miles over old lava flows, to kipukas (land islands that the lava flowed around, leaving unique species of birds and insects to evolve, much like a miniature version of the Galapagos), and to an old utility road.
The reviews that are listed are from people who visited the vent in the Puna district at about 1,200' elevation, and were guided for about 3 miles through forest and onto new lava fields on an unnamed trail across private property by a tour guide for pay.
Both are good hikes, depending upon what you're looking for, but long, rough footing, and hot.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.