I have a crud catcher and it certainly works, you can hear it ringing like a bell at low speeds. So far forks are ok, but I do have a couple of chips on the forks of my 4 year old Bonni so would like to avoid it on the BM
If BMWs were so well build we would not need fendder extenders, crud catchers,ACF50 and a whole host of products in order to keep them in good condtion.
Ride your bike throught a winter or two unprotected and see what it looks like, or more important "What it`s worth"