I have always found CIDER pretty simple to use. Start a REPL using jack-in, evaluate code in the source code buffer, quit/restart when adding new libraries. I used to find wiring external things likke configuring auto-complete challenging, until I just let Spacemacs do that for me. I also find CIDER key bindings much easier with Spacemacs mnemonic keys and Vim style editing. Thats a challenge I have with Emacs in general though, not CIDER specific. I hope to find more time to learn the code base more, so I can contribute back, but there is a bunch of essentials it feels I need to learn about elisp first.