A calcium reactor is something you add when you can not easily maintain calcium levels any other way. Typically this only occurs in systems with large amounts of SPS corals and/or clams. For most other systems water changes, and calcium additives will do the job, and at a far lower cost.
With a calcium reactor, you'll also need to invest in a CO2 bottle, and a regulator with a solenoid valve. This makes the total package rather expensive, so it's only worth doing on large systems, usually about 100 gal tanks and up.
All that being said, they can be a very good addition to your system.
A controller is another nice thing to have, but they do have a high initial investment. If your going to consider one, first figure out what your going to control. You can use it to control just about everything, but also keep in mind that you create a single point of failure. If you loose the controller, you can have a lot of gear not working.
If all you want to control is your lighting your often better off with a few good timers. On the other hand, if your going to control a calcium reactor and similar gear, a controller can do wonders. One of the really nice things you can do with a controller, if you monitor pH and ORP is catch things happening in the system before it gets into real trouble. Also many controllers come with software so your computer can log the readings. This allows you to spot long term trends.
One last point to people considering a controller. If other areas such as lighting or filtration need improvements, you are usually better off spending the money on those, rather than on a controller. If all your other gear is where you want it, then a controller can help a lot. As usual, put your money where your going to be the most cost effective.