I know people who love Eheims LOVE Eheims, the way I love New Life Spectrum. But my experience with them doesn’t support the idea that they are exceptionally superior.
I think many people who recommend Eheims have used them for a long time. If they used Fluvals it was one of the older lines which definitely had some issues. They then switched to Eheim which was a much better filter at the time and they just never looked back. I firmly believe that if those people tried an 05 or 06 line Fluval they would think differently.
For other Eheim fans I think they heard how good Eheims were, went for them, and haven’t had any issues or reason to try anything else. I also think that if these people tried Fluvals (or if you stuck an Eheim label on a Fluval) they would love Fluvals.
I have worked with several different Eheims and have never thought, “I wish Fluval had that.” On the other hand, there are multiple things I would miss about Fluvals if I were forced to use something else (tank rim adapter, intake strainer, etc.).
In general they are high quality and reliable. I am not saying they are bad filters. Some models have better features than others so if you decide to use an Eheim either get a steal of a deal on a used one or do research to make sure you spend your money on one that has all the features you want.
Price, they are definitely more expensive than equivalent Fluvals (or any other canister for that matter).
Features: On the ones I have used there have been no tank rim adapters (nothing holding the tubing to the tank as opposed to coming off and leaking everywhere), bright green hardware (very ugly in all by maybe a well planted tank), bright white ‘Eheim’ lettering on hardware inside the tank, and some models have no sort of tray or media arrangement.
There is nothing about Eheims that can’t be found in cheaper canisters. Although they were once the only company that figured out how to make high quality canisters, that is no longer the case. Now for less you can get at least as good (but honestly others are better).