as someone who has used several sources for carbon dosing i will say it works and works very well depending on what your trying to achieve with doing it.
For mikec who are you getting your info from ?
i would happily point you in the directions of someone who has been using a carbon source for almost a decade, and would say he is one of the reefers who helped the hobby get to where it is.
The same can be said for a tank that doesnt dose or does dose, you will eventually run down to certain bacteria's within the tank that have taken the dominate stand, hence old tank syndrone, which i buy into but only partially depending on how the tank is run and taken care of and what all is used.