If you run sand they are the most expensive filter to maintain bar none. In the three years I have had mine I have been through 12 impeller shafts at 11 bucks a go and 4 impellers at 20 bucks a go. I would get maybe a week or two of good flow before the impeller shaft became noticeably worn. First the flow drops off, then after about a month the noise is so ungodly loud you have to change it. I did get about 3 good months out of it with the last shaft I was using an eheim ceramic shaft which lasted longer.
I have nothing but good things to say about my emperor 400. I have been battling BBA in my tank the algae stopped the filter and ran it dry a couple times while I was at work. Clean the impeller fill her up runs like new.
I got it at the same time as the AC and its still on the first impeller. Now I did get 4 defective units before getting a good one.
I am now retiring the Emperor, The AC is in the trash.
In their place I am running 3 Rena XP3s a CFS500.