As a Java->Clojure guy I find better readability of Clojure's let compared to CL's support of transpose an acceptable tradeoff, but the point of being a lisp is that you are not constrained by all decisions of the boss?
It's a little different if you're either creating a domain-specific language (at that point you've already made the decision to have a new language, so go nuts) or if you're adding new control structures that you can't have in the base language without excessive boilerplate (see for example the somewhat common AIF macro, which binds its conditional result to a variable). But I'd consider making a new MY-LET macro because I don't like what the built-in LET looked like to be a bit gauche.
Regardless, I just decided to stick with the Lisp dialect that already worked the way I wanted.