I've always kind of just taken it for granted that 「良い」 can be read as 「よい」 or 「いい」 and that they both mean "good." I suppose I never really gave much thought as to when/where 「よい」 is used, as 「いい」is much more common in daily colloquial Japanese. Yesterday, though, I was having a conversation with my boss regarding a push by the ACLU to get U.S. companies to move call centers to America, and I mentioned that I think many Americans regard domestic call centers to be of higher quality than those that are offshore. The Japanese I used was 「いい質」.
Lately my boss has been on kind of a quest to improve my Japanese (I'm grateful for his good will and attention, but this can be stressful at times), and he told me that instead of 「質」, I should say 「品質」. All right, fair enough - I guess because we're talking about a (product or) service? Then he said that I should say 「よい品質」 instead of 「いい品質」. Hmmm.
I asked him about the difference between 「いい」 and 「よい」 and he sat back with a smile and a 「そうねぇ」 and thought for a moment. He explained that 「よい」is used more often in writing, but that in this setting (I'm not clear exactly if he meant as in a workplace or a more academic, research-oriented setting) it's sometimes good to use words that would normally be used in written Japanese. I nodded.
Having done a little searching online, it seems to me that some people believe 「よい」 and 「いい」have different nuances (which would make sense), but I personally had a hard time grasping the difference that said people were trying to explain. I suppose for now it's enough to understand that 「よい」is used mostly in written Japanese.
I wonder if there's anyone reading who could share some additional insight...