I come from a rock climbing/industrial access/rescue background originally and have been using ascenders and rope grabs for just over 15 years now. I 've used most brands and models of toothed ascenders and rope grabs. The toothed style ascenders are often blamed for being rope wreckers, but I think they are well below the general wear put on ropes from other things. Most rock climbers are pretty careful with their ropes - the ropes often dont get loaded at all during a climb, and many will lay out a rope bag on the ground so the rope doesnt get any dirt on it at all. Most climbers still retire their rope or degrade it to a secondary purpose after 2-3 years of occasional use.
Industrial access, caving and rescue work are much tougher on ropes. Caving especially is a pretty extreme environment where ropes are continuously subject to mud, water, abrasion over gritty limestone, ice and heavy ascension. Caving ropes don't last anywhere near as long as climbing ropes but again the main cause of damage is the grit and abrasion, not the ascenders.
When used correctly, toothed ascenders place very little extra wear on a rope, certainly less than the wear put on a rope from running through a crotch or the wear from friction hitch heat. They are not meant to used to catch dynamic loads though, and can cause sheath damage if not fully engaged when loaded. They are less likely to properly engage on larger diameter ropes.
Rope grabs (shunt, microcender etc) are less bitey on ropes, but also less likely to positively engage. I trust a toother ascender over a grab to positively engage. The biggest problem with the grabs is when you use them without a sling in between. i.e. some guys attach one directly to their harness with a karabiner. Because there is no flex/twist in the biner, sometimes the arm doesnt get loaded properly. This is mostly fixed by having a sling or cord between the device and you.
Shaun