I think the secret to the treadmill is to have a focus, something to specifically train for a session, not just running to clock up the miles. That would bore me to tears.
I find that running on the treadmill is actually easier on my muscles and joints because it is a nice flat and forgiving surface. Running on the roads or trails here is always a risky thing to do (especially in the dark with no torch!) because they are so rough.
Most of the trails I train on is old and crumbly reef rock, so it is really hard on your ankles.
I am happy to be back running outside though ;-)