People in support of infix in this thread are using "familiarity" as an argument whereas critics are arguing that infix is *one* aspect of the Scheme language that you have to familiarise yourself with.
I'd like to propose that infix (maybe as well as prefix) as a way of thinking and understanding may be rooted in the human brain itself.
I've just read the "DCI manifesto" on Artima, where the authors are arguing that humans are understanding in terms of things and behaviors. Consequently they propose to model "things" and "behaviors" separately.
As a functional language Scheme would be on the outmost "behavior" side of the possible spectrum. Under the above stated theory it would be a compliment to the human brain that it *is* able to manipulate a symbolic problem representation that is very much focused on only one side of "things and behaviors". On the other hand it would hint to why prefix notation is hard to handle for humans - it supposedly simply doesn't match the "natural" way a brain works.
I'm completely ignorant about that topic and respective research and as such could be completely wrong, however I think we should not stop our thinking at the relatively trivial "familiarity" argument but ask whether the problem could be rooted deeper than that.