I like the idea but it's lots of work to set up. Level switches, pumps, overflow drains etc. can and do fail. Then there's the programming in the controller-you want to include some code to account for all possible failure events. Also, your system is open to the sewer? And if that backs up? Checkvalves can leak.
I had a similar system set up on my 180 but I feel that I got better results from a single large monthly waterchange so I went back. Live and learn.
Good info here :
http://www.reefkeeping.com/issues/2005-10/rhf/#4