When I looked around for Common Lisp environments back in late 1993
  so I could program in a programming language instead of killing
  myself with C++, I tried a number of languages, including Ada,
  Smalltalk, Scheme and Common Lisp, all of which have reasonably good
  free implementations.  (MIT Scheme was excellent, but Scheme is one
  of those languages that are OK only as long as you live in a dorm
  room and are unaware that you will eventually want to buy a house.)

  Common Lisp implementations included CLISP and CMUCL at the time and
  CLISP was immature and nearly useless at the time, even worse than
  KCL, which a friend got his hands on sometime in 1987.  CMUCL has
  been excellent all the time I have used it, but I was unimpressed
  with the CLOS (PCL) performance and hence did not work a lot with
  that part of CL until I got Allegro CL.  I still have a complete
  CMUCL system installed on my system and occasionally use it to
  ensure that I know which parts of my code are portable to it, but I
  no longer use it in any "production system" sense.