I have been running a GEO 618 for a few years now. I control it with a Pinpoint controller. I find it to be super easy. Almost set it and forget it. My alk stays rock solid. I give GEO my full endorsement.
For set up you can go without a controller but it can be very difficult to set up and hardly worth the trouble. A controller is the way to go.
A much overlooked aspect of running a Calcium Reactor vs Two Part is that you are breaking down and dosing actual coral skeletons. The result is that you get all the trace elements that those corals took in and used to build their skeletons.