I actually tend to avoid them for my ordinary Lisp code; recently, I even found myself rejecting a third-party library because of its use of reader-macros. This is mostly due to the fact, that unlike symbols, there is only a single "namespace" for reader-macros. And I often seem to disagree with authors of libraries about their taste when choosing an appropriate dispatch character.
However, I used customized non-standard readtables + read successfully for simple parsing tasks. The most complex parser thing I implemented so far using a custom readtable was a HTML template engine (yet another, sorry) with a syntax similar to JSP/ASP, but using Common Lisp as the actual template language, so you can have things like
<% (for (title . link) in breadcrumb do %><%= title %><% ) %>
(wasn't done with readtable hacks alone, though, code had to undergo a preprocessing stage).
