It has to be a reason, i bought mine new and sold it with in 8 months, a lot people say its the best thing and i did belive it but after six months of service i retire the thing and i tell you why.
First eheim 2028 baskets hold more media then the fx5 and made the comparison filling the baskets of the fx5 then put it on the 2028.
Second you can use any polyfiber because gurgle problem it took me a while to figure it out what was the problem, and i remenber coming home after work and the thing was shooting bubles with little flow that was the last thing i wanted to do star playing with it, i seen a lot people in here with the same problem.
So not enougth media for the size of the bucket, not able to use polishing pads, now this its about personal taste, i dont like the intake it looks like a big computer mouse, the hoses specially if you use it on high temp they tend to crack, the plastic know with metal screws for the lid they brake no brainer metal and plastic dont get alone.
A good filter should be versatile, reliable and for that size should hold more media, eheim 2250 with 1260 pump blows away fx5. I know a lot fx5 wil be ofended by this but its true, fx5 its a good filter but no the best i had it and i would not buy onother .