So let me say this outright, these guys make the absolute best ro/di filters and membranes that I've ever used. The carbon lasts longer, removes more silicates etc and so on. I can't think of a single complaint actually, at least not about the product (or the support - these guys walked me through installing flow restrictors etc over the phone). You DO pay a premium, but it's worth it: while this carbon costs about twice what BRS charges for theirs, it lasts longer and seems to prevent diatom blooms even better.
So why only three stars?
Well, I'll tell you. One DI resin cartridge is 20 bucks. I can live with that. What I RESENT is being charged 20 dollars to ship that one DI resin cartridge. It seems they use flat rate shipping which is fantastic if you're routinely ordering 20 or 30 lbs worth of stuff...but for the one off order? Very rough.