My problem with parabolic rollers is the material. Not only is plastic more likely to get out of round than aluminum, but I was riding a friend's plastic rollers on my plastic bike and I kept on getting static shocks any time I touched anything metal.

I like Kreitlers... the old fashioned 4.5" dia drums. I've been riding rollers for years and don't really need much in the way of concentration to stay on and centered. The only times I've almost fallen was watching a particularly good play during a football game. If you set up close to a table (I use my workbench), you get a slight warning in the form of a touch to your hip if you get close to the table side edge.