We've got a 620 and bought it when i had a back problem,a little smaller than 640 but its great for fert speading, rolling, general haulage,and hedge cutting.We've had no problems with it apart from two new doors, not the tractors fault,just watch the gas strut stay on the door as it gets worn after a while.
I would like to change another tractor this year so will definately look at the 640