I used to develop Java on the mac and it worked out well, combo of just using the terminal and IntelliJ. What are good tools that run on the mac for doing Ruby development
I love TextMate although have been flirting with NetBeans. If you like a full-featured (aka bloated) IDE, give it a try. Pretty nice: http://wiki.netbeans.org/Ruby
I'll second/third the Textmate recommendation. Calling it a text editor is doing a bit of disservice. Start off with this PDF that has some useful ruby/rails textmate shortcuts
You have used IntelliJ already? You should probably take a look at the Ruby/Rails integration. It supports things like debugging, code analysis and refactorings, etc.
if you like vim, adding onto it with fuzzyfilefinder (http://github.com/jamis/fuzzy_file_finder/tree/master) and rails.vim (http://www.vim.org/scripts/script.php?script_id=1567) plugins provides a really nice experience.
if you don't like vim, textmate is a decent text editor.
TextMate is my favorite, but if you want code completion NetBeans is the best choice (Aptana/RedRails is outdated). Nightly builds of NB has a lot nicer OSX look and feel.