I used to work for an aquarium maintenance company so I have plenty of experience using carbon, and have a mixed opinion on its use.
I found carbon to be generally a good thing (either a great positive effect or little to no effect at all) with one major caveat - use quality carbon. The only issue I ever experienced with carbon was HLLE. I had been reading the talk about carbon and HLLE and was ready to pull carbon from any tank that didn't explicitly benefit from its use, but before we did this, I noticed one commonality. Tanks in which we had otherwise unexplained HLLE were using Marineland black diamond carbon. We switched one of those tanks to Seachem matrix carbon and noticed a big improvement in the affected tangs within just a few weeks.
Of note, is that the HLLE is more often than not caused by either sustained high nitrates, or by stray voltage. If your water is good and you have HLLE in your tangs, throw a multimeter into the tank before blaming carbon.
I don't currently run carbon on my personal aquarium, but for kicks, I am going to throw a bag in tonight when I get home from work. The main factors I will be monitoring are dust algae growth on the glass, skimmate output, coral response, and my beloved Kole tang.