My current setup does not compare favorably with Emacs/slime at this point. I use screen, I call a build script in one shell REPL, I use gerbil-mode in Emacs in the other, reload'ing things that I rebuild, or restarting the M-x run-scheme when too much has changed, copy-pasting from a gerbil-scratch.ss file, or repeatedly loading a .ss file with on-going tests. This is less than satisfactory. We hope to improve the situation this year so we can use geiser or slime or something similar.