I think it's worth it. I'll do a half charcoal half something else next time. I just had the 2 gallons of charcoal lying around. It's on my 300 freshwater South American tank and BOOM, instant clear!
Oh, email the people, i did and got 10 dollars off with free shipping. And I'm talking about the person that sells them on Ebay. Very legit, very nice to deal with.