Previous review by Sinuhe Reyrub is really good, but I though I could add some information here as my first review.
When planning my visit there I though it would be easy to enjoy the site due to its convenient location between Puebla and Oaxaca. But I soon realized it would be hard and time-consuming to visit this biosphere reserve on public transport. I finally found that ecotourism tour operator based in Oaxaca offering tours there (http://tierraventura.com/ruta-tehuacan-cuicatlan/).
We chose a three-day tour in the reserve starting and ending in Oaxaca, and we had a really great time. It was probably much more expensive than visiting on our own, but it was easy, nice and very instructive. I think this is the best way to visit the reserve if you rely on public transport. We visited (and stayed in) the amazing botanical garden (already well described in the previous review). The diversity of cacti there is impressive. We also spend some time in San Juan Rayas to see the countless fossils and dinosaurs footprints. Around those two regions, the landscape of cacti covered mountains is really beautiful, especially at sunset when the light gives a deeper colour to the canyons. We hiked a bit around and spotted birds, lizards and a fox at night.
On the second day, they took us to Santiago Quiotepec, a very small and remote village. From there, we hiked a nearby mountain to see some ongoing archeological works and enjoy once more the superb landscape. We even swam in the pristine river there. The ruins there were nice to see but are far from any comparison to any other Mexican prehispanic WHS. The natural beauty is more what makes the value of the valley.
On the last day, we visited the Sabino Canyon (already described here as well). I don't know for the evening visit, but the morning is an excellent time to hike the canyon. We arrived before sunrise and macaws were already numerous flying around as they leave the canyon. We enjoyed breakfast on top of the canyon and spend a couple hours there to see them returning inside. In the morning sun, their green, red and blue colours were dazzling.
I can't say from my visit that this site deserves inscription based on cultural criterion, but it is definitely a great natural site.