I do ALOT of remote diagnosis / inspection / debugging. 'lein repl' just makes my local workflow similar to my remote workflow (via ssh tunnels). I'll second the value of having a separate buffer with all the tracing data dumped out to it vs. showing up in Emacs.

I tend to start the repl at a command line and then connect in. That way my repl session isn't tied to the emacs process, and if I do something stupid in emacs I can disconnect and reconnect without issue.

If you use lein repl and emacs crashes, your repl will stay up and you can reconnect to the same session.

Yeah, I crash emacs a lot...

Also, if you run a command that prints a lot to stdout (eg tracing) it will make emacs unhappy. That works much better in the lein repl terminal.