Based on the description on their site, the controller includes a built-in battery: "8.39 Wh Li-ion battery, 35+ hours of gameplay... "
That was disappointing for me. Specially condidering the Steam Frame's controllers make use of AA batteries: "One replaceable AA battery per controller, 40hr battery life"
AA Batteries might not be as convenient to use, but being able to replace them is a great advantage. All my Xbox360 controllers still work fine, but none of my PS3' Dualshock 3s.
The official docking station could be used to recharge (rechargables) AA batteries so the functionality could remain the same.
Rechargeable AA batteries are a thing. From an ecological point of view, the custom battery is a bigger problem. In 20 years the controller will still work and AA batteries will still be available, but no one will be making new built-in rechargeable battery replacements (even if Valve originally intended for the battery to be replaceable).
One of my favourite controllers has been the Dual shock controller. I've got 2 PS3 controllers that I love using. Their rechargeable batteries are shot to shit. I would be much happier if I could use AA.
If the device has a 40 hour battery life and the battery lasts 1000 cycles, that will give you 40.000 hours. That means you can play for five hours every day for over 16 years. I don't think many devices will last longer than that.
Batteries still wear out because of age regardless of how many charge cycles they've had.