I've been happy with my Velo Orange aluminum fenders, and they're so far unblemished after 10 months of all-weather commuting, plus some rail-trail riding. The only problems I've had are related to the minimal fender clearance of my frame (Salsa Casseroll). I'd probably have these same problems with plastic fenders.

However, to be honest I chose these for form over function; I've also used Planet Bike fenders on both a tourer and a tandem, and found them to be very effective and durable.