I have a full set of Speedymoto sliders purchased from CA Cyclewerks (front & rear axle, frame sliders).
Quite happy with them. They are just your usual rubber sliders, but install was easy, they look good and stick out enough that I'm confident they'd do well in a stack (hope not to test them of course )
to answer you question about damage to frame or engine,,,, no, it shouldn't cause any frame or engine damage.. it would really defeat its entire purpose.. most people install them (even on superbikes, its hidden under their fairings),, however some models look too bulky, and therefore discourage riders. I had some rizomas installed,, its the more stylish of the bunch.
There is an ongoing debate about whether sliders can do more damage than good.. this mostly relates to the track though. The argument is that if you stack a sportbike at high speed there is the potential that either the impact of the knobs against the ground bends the frame (depending on where they are mounted) or that the knobs catch on something and cause the bike to flip, doing way more damage than a slide.
However the way I see it I have insurance on my bike, so the knobs are just to prevent damage in a relatively minor drop.. if the drop is serious enough that I need to worry about the above, the bikes probably going through insurance regardless of whether I have knobs or not.