I've been using a minimal amount for the past decade that I change out monthly. One major issue with carbon is that you do not want it 'fluidized' (tumbling) in the reactor. If the media bounces around it will create fines which have been linked to HLLE Head and Lateral Line Erosion.
http://www.advancedaquarist.com/blog...r-hlle-disease Aside from that risk it is a great product to increase water clarity and remove harmful compounds (especially for a mixed reef). Some critics complain that it also removes beneficial trace elements from the water. But for the average person those would be replenished through regular water changes.
ROX carbon seems to be the way to go these days. Its a little more expensive by weight but has greater surface area and is therefore more effective. There are lots of suitable reactors out there. The old Two Little Fishies design wasn't very good in my opinion so I've been using Bulk Reef Supply's for the last few years.