It's not the effectiveness - which I'm sure is fine for what it is - it's the amount per pad. But if your filter needs a pad anyway and you're changing them weekly....probably a sensible option since it cuts down on having a carbon filter to clean/bag to maintain. It's definitely not cheaper though.
Thanks. A friend of mine has ditched filter socks and is now using carbon pad to stop micro bubbles. He just pre cut the whole pad. Not sure how often he plans to change them.