Speaking for myself, I found automating my calcium and alkilinity supplementation to have a huge impact on my tank. Manual dosing works very well as others have shown (Fay eg.), but for me though I just wasn't getting results. Maybe I just wasn't as consistent with my manual dosing as I would have liked.
At the time I considered balling method vs CA reactor. Too be honest I would have gone balling, but a well priced reactor popped up on trademe and I got it. Initial expenses for both methods were about the same from my research.
All up I spend:
$350 - Brand new Aqua Medic 1000 reactor (with media)
$150 - S/H 6.7 kg CO2 cylinder (c/o Wasp)
$150 - Tunze Co2 regulator
$xxx - Bubble counter + non return valve
This would be considered cheap for the setup given what it is. Been running it for about 3 months now and the impact on my tank has been staggering.
If you're so inclined, a CA reactor is exteremly simple to DIY. If I ever have to add another chamber I'll DIY it.
Just bought an RO unit, so can't comment on them really. There have been minor issues with my tank that correlate to using tap water (cyano, hair algae) but have just been putting up with it.
Cheers, Bubs!