I myself did a very challenging hill workout (which i'm hoping will help to prepare me for my upcoming trail race) on the treadmill last week. I stole the idea for the workout from: http://instephsshoes.blogspot.com. Personally, I did not feel it was a waste of time AT ALL. In fact, IT KICKED MY ASS! I should really do hill workouts (outdoors and on the treadmill) more often.
Here's the recap:
Sunday - 7 very chilly miles (8 degrees outside, 2 pairs of pants and 3 shirts.... I'll still take it over hot and humid!)
Saturday - 5.5 Chilly miles (below 10 degrees - but the sun was out!)
Friday - 4 miles easy (on the treadmill BTW)
Thursday - rest
Wednesday - Cardio intervals (on random cardio equipment at the gym) interspersed with mainly upper body weights.
Tuesday - 5.1 miles total with Treadmill Hill repeats as follows: 1%, 2%, 1%, 4%, 1%, 6%, 1%, 8%, 1%, 10% for one minute each. Repeat twice.
Monday - Full Body Weights
Weekly Milage Total: 21.6 Miles