[t420mom]

My personal experience has been that the live coding aspect makes a huge difference to my development speed. A while ago I was going through Project Euler in less-mainstream languages, just as a warm-up/distraction from work. I would do one problem at a time and then go back to work. One day I switched from Object Pascal (with Lazarus) to Pharo. I completed the problem (which I hadn't attempted before, so no prior knowledge) so quickly, that I proceeded to complete five problems in the time it usually took me to do one. Because everything was so immediate, it was really easy to maintain a flow state. I had to force myself to stop and get back to work.

The object inspector is really nice, also being able to redefine methods in the debugger and continue the current computation is super cool. Beats poring over Java stack traces.

It would be nice if there were more open source libraries available, but SmalltalkHub is getting better: http://smalltalkhub.com/list