Eclipse, with the newly released PDT 2.0, is a very capable Drupal IDE. With code completion, automatic documentation lookups, and integrated debugging, Eclipse is very good for anyone who spends time doing Drupal code.
One issue I've run into with Eclipse when working on contributed modules is that the modules themselves aren't located within a Drupal installation. I found myself resorting to external programs or the command line to copy my changes to my development site or to my local development copy. Using Eclipse's External Tools, it's possible to deploy changes with a single click, greatly reducing the time to test modified code.