I have one (that I got as part of used a set-up I bought) that I use to run water to a dump filter (sump on top of my tank). It moves some water but I clean it (rinse the foam media) about every two weeks to remove all of the built up stuff.
My issue with FX5 is my issue with just about all canisters: compared to sumps and even air-driven box filters, they're a PITA to clean....and (unless you're using a pre-filter) they're likely not cleaned frequently enough to prevent becoming filthy.
Also, compared to a sump or dump, there's a relatively small volume in an FX5....which is why I added the dump (an 18g rubbermaid with a tray of fluff and bioballs, scrubbies, poret, etc.) to my FX5 outflow.
I can change a fishroom full of box filters in the time it takes to clean one FX5... I can't imagine having to clean like 5 of them as regularly as they should be cleaned!
Matt