Murfs --- I would say PLEXUS is a good maintenance product and it has other uses besides your headlights. instrument panel/cluster, stereo, LCD computer screens, tint on tinted windows, eyeglasses, ect.
When the lens on my headlights start looking dingy i personally prefer to hit 'em with meg's PLASTX. I wrap a terry towel around a rubber sanding block then with a little elbow grease start polishing them out. I find the plastx to remove a lot of the more stubborn stuff and plexus is a good way to maintain the lenses afterwards. I do the PLASTX thing maybe once every 3-4 months, plexus maybe every other time i wash my car ( i wash it weekly).
hope this helps