O rings was surely the biggest problem with the eheims, and sometimes more often than it should have been... but overall, with ALL filters of ALL types, the problems stemmed from lack of maintenance.
I've never had any problem with bypass in the Cascade filter, and like I said, I've been running the 1500 model on my 220 of 6+ yrs now. I also never had leakage problems or cloudy water problems, and my oscars are pigs. That is the one tank that is lax on water changes because it is at my ex's house... He started that tank with HOB filters, and always had problems with noise, mess, and accessibility issues. The HOB he had were cheap and didn't work well. Since putting the Cascade on there, I've never had any problems.
If I had a choice between the FX5 and the Eheim, I'd still go with FX5, simply because they have more "features", run equally as well, and for a portion of the price. I just was not impressed with the FX5... I didn't find it to be worth the cost, and I don't think Eheim's products are all worth the money they charge for them.
When it came time to stock our parts wall, Eheim was the one that always had the biggest order, the biggest need for stocking. Anything from O rings to impellar assemblies, we were always selling out of them, and people were always bringing us Eheims that were leaking or clogged.
I have an older style Eheim and I honestly don't care for it because it's hard to prime, making maintenance a pain in the butt. I've worked with the newer ones, but never understood why, in freshwater, they were "so much better". I just have never seen any proof of this.
My husband was always an Eheim enthusiast, until I introduced him to the Cascade. We sat with both filters, and I asked him to show me how Eheim was better... and then we compared the 2. What we found surprised him enough that he gave the Cascade a chance... and now it's one of the filters he reccomends to others almost as much as I do. I am currently bugging Penn Plax to do something better with the valves, because I like the way the new Eheim valves pop off in one piece, creating a good seal when doing maintenance... but outside of that (which to me wasn't worth THAT much more money), we couldn't find much for differences in functionability... not enough to justify the cost differences.