Almost seems a silly question... obviously it's easier to run downhill than to run uphill at the same pace, though I accept that running hard downhill can be more discomforting than running hard uphill. Though I still find downhills easier.
One thing I noticed whilst running last night though. If you run down what appears to be a steady descent, and then turn at the bottom to run back up it again, it suddenly becomes a much steeper incline!!! Honestly, I ran down a hill last night, barely noticing it at all, and having looped round to head back up the same hill it seemed SO MUCH steeper, I could barely believe it.
Another hate of mine is running through a valley, i.e. running down a hill that leads straight into an uphill... as you reach the bottom you feel like your flying, before slowly grinding to a near stationary speed on the uphill. Eugh!
Anything bigger than a disabled access ramp kills me. I have recently found how to run downhill quicker by relaxing and sort of "letting the brakes off" which does help to equalise the fact that on the following uphill I look a bit geriatric.
It all depends on the incline.
In one of my usual forest routes I have a downhill section where I seem to be gliding along effortlessly, no matter how tired I am - it's just that perfect gradient.
2 miles later I have a descent where my knees and heels start complaining immediately.