Your individual needs should dictate whether/when you use a full-featured IDE or a lighter weight code editor.
For lightweight tasks, I still prefer SciTE, tweaking the settings and functions to meet my own needs.
For larger projects I use the NetBeans Ruby IDE. I tried NetBeans a couple years ago and wasn't impressed. But they've come a long way since, especially with regards to Ruby and Rails. Nothing against Eclipse/Aptana; NetBeans just seems to fit me better.
Textmate is very popular on Mac OS, and E (not free, but inexpensive) is the closest thing to it on Windows, and supports TextMate bundles. It seems to have gained many fans.