i've got an acerbis 1 on my trusty ktm. it's made of plastic, so i doubt it would do much in the event of a serious rock hit.
i read the comments in db, too, & i'm sure they mean well, but i disagree. the cons to the guards are: they tend to collect mud & hold it in your brake/hub area; & it takes a few extra seconds (which can matter) to remove the wheel/work on the brake, etc. but they have 1 important benefit: ever blown a turn or gone off-trail to make a pass or "hot line" or miss a bottle neck & gotten a stick or brush caught b/t your front rotor & pad? it can make you stop in a BIG hurry & it can take a while to dig it out.
ps this might get moved to product reviews.