I can't answer yet, because I have yet to get one, but my landlord requires that I have a tractor instead of a stationary coop so I'll be following this thread!
One thing that I'm not looking forward to is having to go out in the muddy back yard to move the thing around. (We have nice grass but it's so rainy that it doesn't stop the mud from squishing up between the blades!)
What style of tractor are you going to build?