The trouble is for everyone person that has had good experiences with fluval filters it seems that someone else has had a bad experience with them, and its the bad experiences that stand out.
Personally i've owned 2 Fluval externals, both 304 models and both of them were nothing but trouble from the first time they had to be cleaned, difficult to restart after maintainance, leaked from around the seals and the double taps and the motors would stop working for no apperent reason, both of them met their demise at the bottom of the garden after being kicked down the stairs and out of the flat.
On the other hand i own 7 eheim externals and have never had any problem with any of them, statistics speak for themselves.
I'm sure there are plenty of people who have had the opposite experiences though, so not really "statistics" but personal experience.
I personally wonder how much of Eheim's positive regard in fishkeeping is from the factor of price tag = popular.
I saw an article written on another board where someone ran the FX5 against the Eheim equivalent and found they were both much the same in performance, ease of use, etc, with the Fluval obviously coming out top because of the price. And yet some other people I've spoken to think something that costs more must be better, with little experience to back it up (not talking about anyone here of course).