I used to use one towards the end of my combustion days. I first had a Roor CFA and then the standard McFinns one. It did work pretty well with keeping my tube clean, and I found the smoke to be more light than when you're completely sans CF or even when you attach an AC. What I didn't like about CF's was 1) having to wash the carbon and then letting them dry, takes a while for them to dry and they never get 100% clean IMO, 2) sometimes the carbon would fall out of the adapter and end up in my water...that always sketched me out, and most of all 3) you need to be really careful with ash/combusted flowers going INTO the adapter during the pull. Nothing like hot burning material burning your carbon...tastes/feels terrible. One way around this was to top the carbon with some shake, or even stems.
I've never used one for vaporizing, mainly cause I don't really see the point and think it's overkill. Vapor by itself is much "cleaner" than smoke will ever be. I also don't think vapor causes the same residues as smoking at all. I don't think there is a residue at all actually. I once didn't clean my glass for a month (got super lazy this summer lol) and didn't notice any resin building up like you find in those bongs that haven't been cleaned for months on end.
If you're someone who combusts, than I think the CF's are worth the hassle. But for vapor thats already clean, and lighter, i don't think it is.