user.clj is also a good place for require’s, so you will not have to reissue them every time you open the REPL.
But better dev/user.clj than src/. The REPL finds user.clj on the Java classpath. Therefore, there should be at-most-one user.clj on the classpath! As a rule of thumb (and definitely, in any project you might share or reuse as a library in another project), put user.clj in dev/ (not src/). When you make a jar or refer to the project with lein checkouts, dev will not be included and user.clj will not become an unwelcome stow-away in your future REPL experiences.