When using Python, the REPL does not feel remotely integral to the experience. For example, my Django project, when an exception occurs in debug mode, has a nice page with a backtrace et cetera. However, I cannot resume, or poke around using a REPL, or even view details of local variables. I feel like if Django was implemented in Common Lisp this would not be a problem at all, if not for technical reasons then for cultural reasons.
If you install the django-extensions package and use runserver_plus instead of runserver, when you hit an exception you get a live REPL in your browser where you can poke around and inspect at will. It's not CL-level power, but it's mighty useful.
For non Django code, the IPython REPL with the %pdb magic enabled drops you in a ipdb debugger on an exception. Doesn't allow resuming but still very useful.
