We used Procyon Common Lisp because it was very nicely integrated with the Macintosh - especially for graphics - and had a great CLOS implementation. I used it to reimplement a clunky VAX-based FORTRAN modelling environment that had been developed in-house into a smooth Macintosh app with a graphical node-graph editor. In all of the system development I've done, this had the biggest 'awesome gosh wow' reaction I've ever received from the users.

I have used it, too - but not for long. Usually I thought MCL then was better - Apple also bought it and then released it via their developer channel.

Unfortunately Procyon Common Lisp was taken off the market, when it was bought by Franz, Inc. They used the Windows version of Procyon CL as kind of a starting point for their Windows offering of Allegro CL, IIRC.