Yes, the buses from the partner hotels is free to the parks/train station. One bit of warning though - it gets super crowded, to the point where it will anger you. There is one bus that is shared among 5 partner hotels, and those hotels as you can imagine have a lot of hotel guests. During peak times, the buses will come more frequently (about once every 10 min or so), but off peak times you'll get one about every 20 minutes. The other issue is that the partner hotels are much further away (I think about 1.5 miles or so?) and it's not a friendly walk, but doable.
Comparing that to the actual Disney hotels - they each have their own shuttle, and it also comes more frequently. Everytime I would be waiting for the shuttle for the partner hotels I sort of regretted not picking a Disney hotel (obviously comes at a higher cost). On top of that, the Disney hotels are much closer, and even the lowest tiered one is walkable (it's halfway the distance to the partner hotels) if you so chose to.
Hope that helps.
