Can't give any definitive results yet, but i like it so far. It just looks like it works well. nice and porous. Im not expecting nitrate lowering, it would be nice in my 300, but flow rate is WAY too high for that. I ran into a situation where my tank was new, and i had fish that needed to be in it ASAP, so I bought like 2 gallons of stability, and have been using that with prime and WCs to cycle it. Its been going great so far though. Seachem makes a good product. I wouldn't expect anything other than positive results from them.
Thats a pretty good deal if it includes shipping. I got my 5.3 gallon from custom aquatics, and shipping was murder. 86 for the matrix, 45 shipping. I probably wont deal with custom aquatic again though, because they do the drop shipping thing, and when 2 weeks went by with no matrix and 45 dollar shipping, I think most rocks Ive encountered were friendlier than their customer service. They wouldn't even get me a tracking # from seachem, and wouldn't return any calls. I eventually talked to a seachem guy, and had a tracking # as soon as i asked him, and the matrix in 2 days.
If you are looking for possible nitrate reduction, it would be better to line the bottom of the sump with matrix. This way water will flow above the matrix and not through it, greatly reducing the water flow that circulates though the media.