Having used a small deioniser filter as part of a wire edm machine recently, which needs a continuous supply of pure water, the resin exchange mechanism certainly seems to work. I can't comment on running costs as our use is very different but it seems that you can reduce running costs by starting with something like rainwater to reduce the "load" on the resin, and a cheap TDS meter is very useful to monitor water quality coming out, so that you know when the resin is getting exhausted. Our filter uses a refillable filter cartridge which helps reduce cost but it rather depends on the quantities you are taking about.