In the end I don't think it's so much about reading level or complexity. I mean there are people that get off on a barrier to entry due a difficult reading level because it makes them somehow feel elite, but those folk are a passing small minority I think. What it really comes down to is, understandability/clarity, ease of use as a reference, and is it interesting enough to hold my interest as a read. Now there are some trade offs involved in those categories, and preferences may vary a little from person to person, but ultimately it takes a good author to write with complexity and retain clarity, and it takes a good author to write with simplicity and retain a sense of depth and interest/engagement with the reader.
I couldn't tell you what grade level something reads at, other than this is harder or that is easier, but I like to think I can tell what is good writing and what is not. And complexity or simplicity aren't, either one, inherently bad or good, they're just tools in a tool box. Authorial voice and the relationship established with the reader are all that really matter, and these are independent of the writing level, though extreme levels of simplicity or complexity can effect the reader's perception of what the author thinks of himself, or his or her audience (often in a negative way).
