When I get on the treadmill I can never decide it I should do it for a specific time, distance, or speed. I guess it's all about how I feel that day and I mix it up so I don't get bored. One time I will say that I am going to walk 5 miles and then the next time I will set a goal to walk for 45 minutes.
I think the treadmill is a very effective workout when you can maintain your pace. Walking is supposed to be the best thing for us.
I have never tried the rowing machine and I hate the climbers/stair steppers since they kill my knees.