I went with the double coil design as I believe it will be the most efficient of the homemade designs.
I've seen them with the coils all touching each other - but to me that is a waste of the copper since every bit of pipe not in contact with the wort isn't cooling it, so the best compromise is slightly fewer coils, but not touching each other.
Given the physical limitations on size - i.e. it has to fit within the boiler - having two sets of coils is going to almost double the available heat exchange surface area - so for every litre of water used you will get a better cooling effect.
I see that as two big advantages - firstly water use - with only a moderate water flow, the cold water going into the chiller was being heated by 60c in the 5 seconds or so it took it to get around the chiller and back out :shock: - that's a very efficient use of water - so if you're on a water meter, any increase in efficiency can only be a good thing.
Secondly, time - having gone through all the time it takes to do an AG brew, do you really want the cooller to be slow?? Get it chilled, get the yeast pitched, lid on, open a beer and relax!!! :drink:
Lastly, the only commercially made one I've seen was a stainless steel single coil one, for around £40 or £45 - for the same money you can make your own double coil copper one - and I believe (could stand to be corrected though) that copper is a better heat conductor than steel, further improving the efficiency of the copper cooller