Here's my thoughts:
Recently I read a thread claiming that ON's label claims would have to be decently accurate. The reasoning was that they are such a big company, other companies would be testing them often trying to catch them up (and therefore increasing the market share for themselves.)
I think that is pretty realistic. When your mass producing something such as ON is, you can afford to sell it for cheaper......that's just a part of business....mass production = cheaper costs for the company and the consumer.
Which also allows them a decent amount of money to spend on advertising....
ON is a much, much bigger operation than alltheywhey or proteinfactory. I have never used proteinfactory so I can give no opinion on them, however all I've heard is good things, besides the amount of time it takes for it to be shipped.
Alltheywhey I used for a year, and I loved it. I went through 5 or 6 5lb tubs of the flavorless isolate. Good stuff.
ON I just recently started using and it gives me gas. Which is a minor annoyance, but still an annoyance.
I'll prolly be switching back to alltheywhey after this whey is gone. All the whey isolate is expensive as hell (something like $35 a 5lb tub) so that is the main reason why I tested out ON. I think the money may be worth it though.