Update on our FF experience...
Been using the simple recipe (feed, well water) for about 6 weeks now. The birds do seem to like it. I block access to their dry feed until after I feed them the FF in the morning. Only feeding the ff once a day. Will probably cut out the dry feed once the weather warms in spring, otherwise their ff will freeze solid before they can eat it all. Takes about 24 hrs for the ff to start to bubble, using just enough water to make a goopy sludge. I don't stir it. Our feed has fish meal and it smells a bit after about 6 days.Nothing horrible. PH is about 4.5 after 8 days, which is ideal (according to the research), and the birds seem fine and they like it. Poops are more solid. I haven't done the math yet, but eyeballing it, it seems we are using slightly less feed, probably actually quite a bit less since they haven't been foraging since we started the FF 6 weeks ago due to weather.
For about 6 months, I've also been making gallon batches of FF using a starter culture. I leave it to ferment, without oxygen (the starter works best without much air) for 2 weeks or more and the pH gets down to about 3.3. The lower pH signifies the presence of much greater quantities of those good lactic acid bacteria (probiotics) as well as creating a fairly shelf stable feed supplement, one that can be stored in a cool/dry/dark place for weeks or even months without loss of nutrients and should improve in nutritional qualities if anything. Combined with the extra effort/cost of making FF this way with the (unknown to me at present) likely TOO low pH of feeding 100% this type of FF over the long term, I will keep feeding this stuff as a daily or weekly treat. I know for humans it would probably not be ideal to eat a ton of pickles, kraut, kimchee, etc (assuming these were low salt versions) simply because the very low pH is usually best as a condiment and not the main meal. In other words, most folks can benefit from some form of lacto fermented food, but you can get too much of a good thing! Same thing I'm thinking for the birds.
So, long story short, the level of acidity in the simple water and feed method were mostly talking about here on this thread seems fine and appropriate for long term feeding. This was a question I had and like finding answers from two sources...from first hand observations from folks who have been doing it a while (like Bee) and also from scientific research. I'll admit that I tend to give more weight in value to first hand experience over the research, but I love combining different sources of info like that. Maybe someone else will appreciate it as well.
Here's one of the research links I found useful:
http://www.ncbi.nlm.nih.gov/m/pubmed/19373724/