I have.
I had to seriously hack on Weblocks just to get things to work they way they should. I had to work around various bugs and omissions. And worst of all, every once in a while I discovered that I am clearly the only user of said software.
A good example is when I discovered that the database interface (to CLSQL) isn't thread-safe. It was a quick hack intended for a single thread only. Obviously you'd never notice it in a demo setup. Turns out I noticed it just when somebody else had that problem and wrote CLSQL-FLUID.
But there is more — after running the application for several months I discovered two things:
a) there are obscure bugs related to caching in CLSQL, which I don't have time to hunt down (stale content is being served),
b) there are bugs somewhere either in CLSQL or Weblocks that blocked part of my admin interface and they are obscure enough that I don't even know where to start looking,
c) every once in a while SBCL will crash on me and land me in the LDB.
Don't get me wrong — I like CL and I invested a significant amount of work in to Weblocks — but I agree with Brian here: I don't want to discovere somewhere along the way that I'm the first real user. At least not when I am on a deadline.
I do most of my work in Clojure now, but when there is a simple web application to write — hey, we get our hands dirty and we do it using PHP. Because life is too short.