Face down Zipline wasn't too bad. I'm terrified of heights so I was basically sweating profusely until the moment I flew across the Fremont street experience. It's not too scary, perfect if this is your first Zipline; great starter. There are no stomach dropping parts at all. If you're like me & are afraid of heights, you'd still be scared throughout the whole thing because you're well above the ground. But you can hug your body & close your eyes to help LOL. I would recommend night vs. day since you are surrounded by all the bright lights.
Knocked off two stars because I think $40 is a bit pricey for this thrill ride especially if you're a thrill seeker since this ride really wouldn't phase you & you'd be underwhelmed by the whole thing. My cousin whom I was with is pretty adventurous (will pretty much ride anything) didn't seem too excited after it was all over. The wait is pretty long too & when you're buying your ticket it's at least an hour + suiting up + waiting upstairs for your turn to be strapped onto the ride. My cousin also complained about how bad the straps jerked when coming to a stop. That didn't happen to me but she mentioned how much it hurt.
I'm half/half on whether if I'd recommend this to anyone which tells me that it's just mehhh.