I've found the Giant guards good but I've not had any heavy rain with them yet... Road spray and standing water will be the real test for them!
I have found them quite noisy, over this Christmas break that's something I'm going to try and resolve as much as possible - first step will be putting some dampening on the metal parts through the forks to avoid any vibration. Next thought is adding some rubber washers to the 'fork supports' to give them a little more flex while keeping them firm. Any other thoughts? What did you try and what did you replace them with?