The intake strainer is the best on the market. It is virtually clog free. The FX6 intake is huge, but very good at spreading out the massive flow. The 406 and smallers have an anti-backflow ball in the strainer, but I haven’t found this to be necessary so I actually remove it to help increase flow.
The tank rim adapter (holds the hoses to the tank’s trim) is very good. It is a nice, tight fit plus uses suction cups. Then the top half comes down completely over the hose to lock it in place. I have not seen another tank rim adapter that is as reliable (some don’t have any at all, they just hang on the tank).
The trays on the 406 and smallers have grating that is fine enough to use big carbon pellets loose in the trays instead of in media bags (much better). I have used Acurel carbon pellets and at least one other for this without any issues.
Unlike most canisters now they use deep trays without a tube running down through them. This is good because you can cram in filter floss very tightly which works as an exceptionally good fine mechanical/polishing media. This also means you don’t have to deal with the tube running down the middle of every media tray.
I think the most annoying thing about the 06 line is the color of the hardware. The 05 line used dark gray/black hardware inside the tank. For some reason, some blind guy who doesn’t own an aquarium decided it would be a good idea to change something that they were already doing right. This means that the 06 line has a translucent white/clear plastic for the hardware inside the tank. This looks hideous, especially against a nice black background. It does actually get a little better as algae and a biofilm develop on the hardware, but it is still completely irrational for them to have made this change.
The impeller cover clip is very fragile. This is 50% better in the 06 line because they changed it to only use one clip instead of two, but this still leaves a chance of it breaking if you open it up to clean the impeller. For this very reason I keep an extra on hand in case a client’s breaks while I am doing maintenance and I tend to skip messing with the impeller cover altogether to avoid the risk of breaking one.
No handles (except on the FX6). It would be nice if the smaller models had a handle, but they don’t. Not the end of the world, but it would be a nice improvement.
I think many people who don’t like Fluval haven’t used the 05 or 06 line. The old lines definitely had some issues, but most have been addressed. So if someone used an 03 or 04 Fluval and then went to Eheim or something else, they may swear by that other filter now and tell everyone that Fluvals are junk. The problem is that they just don’t know how much better Fluvals are now.
I have used and worked with Eheims, Renas, SunSuns, Marinelands, and most of the others and although not perfect, I have definitely found Fluvals to be by far the best. They are worth the money. You won’t regret buying one. If you have a chance to buy an 05 line instead of the 06 line, take it. The hardware color alone is worth it, but almost nothing else changed between the two lines so the cost savings should also make it more than worth it.