It does the job but I have the feeling that PhosGuard does not remove as much phosphate as some other competing products.
I would not recommend using this product because it use aluminum oxides to remove the phosphate. It does work, but isn’t an ideal product for a reef tank. According to Advancedaquarist.com, it often leaves enough phosphate in the water for nuisance algae to remain a problem. The other issue is the presence of aluminum in the filter material. It has to be washed to remove the aluminum particles, but even if washed it can increase aluminum levels in a tank (and who wants that?).
For more information about aluminium, have a look at Aluminum in the Reef Aquarium