I have the SW Motech brand sliders and I like them. I did manage to drop my bike while turning around in a parking lot last year and I can verify they do work. My right side slider pucks are chewed up and definitely took a good hit when the bike fell over. They are not really to protect your axles per say but they absolutely do protect your brake discs, calipers, differential (rear pumpkin) and the lower part of your forks. When my bike fell, the crashbars, axle sliders and the very end of the handguards were the only thing that struck the ground and got damaged. Long story short, I am glad I had the axle sliders.
Ebay carries the T-Rex brand of sliders that look very well made and cost a lot less than the SW Motech brand that I have. I would probably order a set of those and give those a try before I dropped the coin on the SW Motech brand. Here is a link to the front axle sliders, I am not sure why they aren't listing a rear slider set, Ebay sells them as a complete set, just a do a search.
http://t-rex-racing.com/catalog.php?item=711
