I was a long time roller user, but I just didn't feel like I got enough out of it to be worth my time. A trainer is much better for me.

I have gotten awfully close to riding off the side of my rollers when watching tv. I usually wake up in time. Not sure about parabolic rollers though. I feel like going off the rollers isn't that big of a deal most of the time. I always have something to grab on one side of the rollers.