Common Lisp guy gonna chime in here...having a REPL that integrates with your editor (vim/slimv in my case) is an intense improvement on any other language I've ever used. With C you edit, leave, `make`, run. With PHP you edit, open browser, F5, run...same with JS.
With lisp, I edit, eval (simple command in my editor), see result. If I'm building a game or a long-running app, I can redefine functions while it's running without doing a full recompile and having to try and get back to the state the app was in when I wanted to test my change.
So you get the ability to live-code while the app is running. Once you get used to being able to do this, leaving your editor and opening to shell to test your app seems like a barbaric exercise.
So a good REPL (lisp or not, doesn't matter) can make the experience a lot more interactive and experimental that your average write -> save -> compile -> run cycle.
I'm no Javascript advocate, but I just found out that you can use Chrome like that for JS, where I can edit a function, press Ctrl+S, the Chrome REPL will say something like "functions recompiled in 80ms," and then your running app will use that function. I have to say that developing like that is pretty great.
Unfortunately the OCaml toplevel pales in comparison to CL environments like slime in terms of visibility and debuggability. Redefinition is also off the table, as OCaml is a very static language. (You can rebind the same name, which can be helpful in exploratory programming but is not at all the same thing.)
