I have used them and they do tend to fill up with detritus and need more maintenance than bioballs, but they are cheaper. If you have space in your sump for more, that should help them equal the performance of bioballs.
I use them in conjunction with lava rock.
Just my opinion but I think they have more surface area then bio balls but tend to catch larger particulate matter.
Definitely best in the last stage of filtration as bio media. Cheap and effective.
As Old Newbie and DJRansome suggested 4 pack at dollar store.