I think it depends on what you want it to do and what size system you have. Would I run one on an 8g nano? Probably not. But if I have a Ca reactor and lots of stuff on a large system I would definitely run one.
They have discontinued the Apex Lite so unless you get a used one, your options are the Apex Jr. or the full blown Apex. I would start with a basic Apex setup and learn the controller and programming before adding a bunch of stuff to it. That would be the "brains" of the Apex, display module, Energy Bar 8, temperature probe, and most places will sell it packaged with a pH probe.
My Apex serves as a back-up and failsafe for a lot of things in my system. For example, I have multiple float switches tied to my Apex to shut down my ATO system. Same thing with a salinity monitor so I know what is going on and as a back-up to ATO and multiple float switch failures. The temperature monitoring also acts as a back-up to the built-in thermostats on my heaters and chiller. Leak detectors can buy some peace of mind as well as the ability to be notified of a power loss so you can get back to do something about it.
In terms of convenience, I use my feed modes on a regular basis for not only feeding but for shutting down certain items when I do my water changes or other maintenance. I use a WXM module to control multiple Vortechs. I also use a bunch of reed switches tied to my cabinet doors to automatically turn on and turn off 3 cabinet lights. I'm sure others can chime in with what they use their setups for.
All that said, if the Apex were to die, I could still run my system by just plugging everything into regular outlets as I have a separate ATO and dosing controllers. I am always paranoid about a single point of failure and the Apex is, like any other piece of equipment, something that has the potential to fail. So I guess my Apex is an integral part of my system for monitoring, convenience, and redundancy purposes.
