I, myself, really dislike &aux. It has been so long since I have seen it that I have forgotten that it even exists. We never use it; and I should add that to our style guide.
I don't even like
(let (a b c) ...)
since I prefer to think of "let" in the Scheme sense of "I am naming the result of a computation." In fact, I added a macro to our library that is like "let" but does not allow the variable to be modified. (It's not portable; it uses a CCL-specific feature.)
Unfortunately, we do not use it in practice, since it's so important for code style to be consistent and it would be very hard to change all our sources to do this, since it's hard to know from looking at code whether the "let" variable ever changes, and we have like 700MLOC of code in QRes. I wish Common Lisp had always had such a feature.
I'm not saying there's no place for let followed by side effects. It's just not the way I usually think of it.
Also, putting things in the parameter list that aren't parameters just feels weird.
I do not think my own opinions are by any means "right". These are just my biases and preferences.
-- Dan