It was very effective, although my fridge was too large for the cold plate. I believe this is the most reliable type of fridge - I never had any trouble, and the unit had lasted for 20 years without failure. That must say it all, really.
The biggest drawback is the obvious - no engine = no cold. This makes such devices useless for marinas and moorings but ideal for long passages etc. I believe some boats have both systems installed.
I fitted a new Waco fridge at the beginning of this season which has a holding plate system which charges whenever the engine is running. If the battery voltage drops below a certain level the compressor stops running and the holding plate takes over. You can also switch off the compressor and run on the holding plate overnight if you want to conserve battery. About 1 hours engine run gives enough in the "plate" for about 8 hours without compressor holding the fridge body to 5 centigrades and keeping the ice hard. It's actually wonderful. I dont know how it works but it does and it does away with all the complication of an engine driven compressor.
The web site below has all the information you need to do your own. If you decide to go that way, purchase his book - it is well written. I used it to make my frig/freezer and it has been working for 12 years now.
Don't buy parts from him, however. He sold me some bad junk that caused many problems - go to a local refrigeration supply source.