I am not using one but they are my next filter to install on many of my tanks as extra filtration .
I am big on mechanicals and sponges for my bio ..
I have strong opinions on all the super special medias ..
When read Dr. Steven Tanners info on Porret sponges then you might fell differently on using the many contrived medias that are for sale with no scientific info to back them ..
I have used aquaclear sponges X2 in my HOBs with none of the media they supply and my sump for my 180g has been run on sponges for about 9 years now ..
Mechanicals and sponges are the easiest to maintain IMO ..
Most other medias clog and or become nitrate factories ..