IMO and many others, canister filters are a poor choice for marine tanks, they trap and harbor detritus and concequently need to be cleaned every 2-3 weeks to run effectively. This is especially an issue when you have these G series Fluvals that are only pushing 300gph. You can't add on a skimmer to them, and they don't provide much oxygenation at all. No where near what a wet/dry filter can produce.
Fluval has never marketed toward marine enthusiasts, the have been exclusively freshwater for years. These only thing these filters have are a digital temp probe and conductivity meter, so what? Conductivity essentially useless, conventionally used to measure the amount of organic build up in the system, in a marine environment it's so bombarded with inorganics that it's hardly useful for anything, except a relative measurement of your salinity.
Second the firmware on the thing is not upgradable, doesn't sync to computer or ethernet connection. Finally the reminders you can set on them have to be done so manually through an interface menu that is a real pain in the ***.
Pros:
They seem to be well built with nice hardware, and changing cartriges is really easy.
Not worth it IMO. We have a Hagen rep at the LFS that I work at, and I like to bust his balls. Being an industry rep I expect alot more out of them as far as product knowledge, and competitor knowledge goes.