My first introduction to lisp was in 1988. My text editor of choice at CMU was GNU Emacs and I copied/modified a few Elisp extensions to assist me in working with it. I also used BatMail (an Elisp MUA for the Andrew Message System written by Miles Bader) - and it used the AMS FLAMES filtering language for re-filing messages, like all AMS MUAs. FLAMES was based on S-EXPs, so that kept lisp fresh in mind.
In 1990, I took a programming language class that taught me Scheme. This was a awesome experience -- despite my less than stellar academic performance (it wasn't due to using Scheme). At this point, I realized that lisp family languages more closely matched my natural way of thinking about problems in computing.
I finally began to dig into completely understanding Elisp for the first time in early 1999. I had begun using Gnus as my mail/news reader of choice and was seriously interested in modifying it. In the process, I stumbled across Common Lisp for what seemed like the first time. Ironically, I'd had CLtL1 in my posession for years, but hadn't every really read through the first few chapters. My interest in Common Lisp was piqued - but not enough to sustain it. I reverted back to working in Elisp and (occasionally) scheme.
It wasn't until early 2002 that I finally dug into Common Lisp in any serious manner. I spent a good amount of time working on it during the first half of 2002, but then was interrupted by the ATHY bankruptcy issues. Just as I was reaching the point of having the time to work on it again while being employed at FASTNET, bankruptcy struck again. Finally, we have reached a point at US LEC that I think I can begin to devote some time to picking up Common Lisp again.
Since I've switched to using Mac OS X as my primary platform, my primary Common Lisp platform has been DigiTool's Macintosh Common Lisp (MCL). I've recently begun looking into running either CMUCL natively or using SBCL (a derivative of CMUCL). I could also look at using LispWorks from Xanalysis, but at this point, I'm not sure which platform is going to provide the best environment for Mac OS X programming.