These reactors are very useful. They're also very similar and add to those one made by Kent. I personally prefer the one made by Kent because of the configuration of the connections for the tubing that they need, but it's not really a big deal. There are even others like the one made by Aqua-C which is different from the others in that it's bigger.
You can use these reactors for carbon, phosphate-removing medium, etc. It's best not to mix the mediums, since most phosphate-removing mediums are designed to be fluidized in the reactor while carbon is not.