I bought Nashbar parabolic rollers for my wife, and I have ridden them a number of times. They are only raised towards the outsides, and are flat in the middle. The benefit is to keep you from going off when you start daydreaming. You get strong feedback as the tire starts to roll up and then slips down the parabolic edge. It isn't enough to unbalance you though. I think they are a good idea for watching TV or otherwise vegging.

As had been posted many times, there is no reason to buy a resistance unit. Just let air out of your tires and/or put an old towel/carpet remnant under the rear roller. Works perfectly.