How widely is NetBeans used by the RoR community? I'm (purely) guessing not very widely at all.
I've never used it for any Rails development, and found the coupling of the Ruby packages with Rails gems, and the presumption in the add-ons that any Ruby project was a Rails project, a bit dopey. There seems to be no way to install the Ruby stuff without the Rail stuff, though.
I much prefer using vim for development, and while there's a vim plugin for Netbeans I typically did serious editing in an external vim instance while using Netbeans for GUI stuff, or for some refactoring, which it does quite well.
For people who like IDEs, Netbeans is a really good Ruby/JRuby editor.
I would say, not a whole lot according to their trend analysis.
Anecdotally, I don't know of a single RoR developer who uses Netbeans. Vim, Textmate and Emacs seem to be the editors of choice.
That being said, emacs is my day-to-day environment, Netbeans was never more than an occasional side tool.
Features I make use of: project navigation, file navigation, syntax highlighting, jump to a declaration, re-indent and very occasional code-complete (mostly for css)
Features I don't use: everything else. I always have a terminal window open with about 5/6 tabs from which i do all my generations, migrations, console, server etc. I use RVM and NetBeans doesn't seem to play nicely with that
Works with JRuby too :)
Just start your Rails server with --debugger and put 'debugger' anywhere in your Rails code that you want a breakpoint at.