Albert R.wrote a review of Platypus Bite Valve Cover on January 15, 2010
First, install is easy. Pull off the bite valve at end of drinking tube[pull out from tube], slide this cover on, then push the bite valve you just removed back into the drink tube, slide the cover back down around the bite valve [a little tight wedging it down but gently nudge it till it fits], and snap it closed.
As to utility, seems the right thing to do to keep dirt and debris from mucking up your bite valve when wearing your hydration pack. Looked good after it was installed. If cap is fully pressed into the lower part, it sealed well.
Unable to field test it yet, but see no reason it would not provide adequate protection.