I've ridden Mantis numerous times, sometimes it was smooth, others it was not.
I remember the first time I rode it I had to wait in a ridiculously long line and I had to use the bathroom badly, so I wasn't concerned about the pain at all, just the fact that I had to crap really bad. lol But the second time I rode it I distictly remember it killing my calfs for the rest of the day. But there have also been times where it has been a relatively smooth ride with no major issues. There was also one time where I was dared to ride the whole thing sitting completely with all my weight, feet off the ground, on the thingamobob that goes between your legs. I DO NOT recommend doing that, it hurt certain places in ways that I never thought it could be hurt. haha. Usually it is a smooth ride though.