On the other hand Quicklisp has serious issues:
+ Minimal if any documentation of internals.
+ A substantial chunk of the codebase can only be described as spaghetti code. To make matters even worse, most functions lack documentation strings. A sad state of affairs given the interactive and self-documenting nature of CL.
+ Is vulnerable to man-in-the-middle attacks since it verifies neither certificates nor checksums. This means that using Quicklisp can get you owned. Unacceptable these days.
+ Operates over a 'curated repository' model that Xach is managing. The repository has been found to be vulnerable to man-in-the-middle attacks in the past since packages were fetched over plain HTTP or git://. Even if that wasn't the case, it's yet another element in the chain of things to trust.
+ Few if any people besides Xach are working on quicklisp-client to mitigate these issues. Lack of documentation and spaghetti code is not helping to attract new talent.
+ Xach is employed by Clozure Associates. I understand that they've allotted him some time to work on Quicklisp while on the job, but it's still a side-project. Look at the number of open issues at Github. Many have been there for years.
For these reasons, I don't use Quicklisp myself. I also advocate against it to friends and colleagues who share my views on software distribution mechanisms.