I used one for a few years. IMO if you're doing reef or FOWLR, you don't need it.
I don't think Kent is making them anymore - production issues with the ceramic used to make the rocker device.
There were a few issues with this filter - the track that the rocker sits on will wear, and eventually .... you'll go off your rocker :lol: (sorry - couldn't resist that!)...I kill me...
There was a lot of splash too. I had an earlier model, used a small piece of plastic shelving to keep the splash contained. I think they modified the design after this, but that made it tricker to service.
I'd give it about a 7 out of 10. However like I said if you're using live rock, cut to the chase and just get a sump. In fact the sump that comes with the rocker is sweet - just take out the rocker Of course I don't suggest spending a ton of money just to get the sump - but I eventually took my rocker out and just used the sump.