These are really comfortable and warm just like a house slipper should be, but have a hard sole so that you can walk outside with them too. I ordered my regular shoe size and they are a little difficult to get on, but fit well.
The spout mechanism is really convenient. You can leave it "open" and it won't spill unless you squeeze it and when you want to make sure it "closes" you turn the top rather than pull on the part you drink from. It's a great design.
So far I've only used it on my road bike on fairly smooth roads. You really have to tighten all the screws to keep it from shifting. I can't imagine using it on a mountain bike trail. The required tightening would probably cause the plastic to snap.