Recently I went shopping for a new IDE. I was developing on a 1GB Linux machine using the Aptana packaging of RadRails on top of Eclipse. A new Aptana release came out that pushed the RAM usage past 1GB. I was also getting increasingly annoyed at RadRails - autocompletion was more of a hindrance than a help, search was poorly designed, and weird behavior resulted on long lines. Nevertheless the level of Rails and Ruby integration was nice - there was no going back to VIM.
My trip to the Google open source shopping mall yielded a few positive reviews of the Netbeans IDE Beta 2. I installed it and there's no going back. After about a week of usage I'm still exploring NetBeans functionality so there may be some things I'm missing, but here's what I like about Netbeans so far:
1. Autocompletion works. For rhtml, for ruby, for css. It helps rather than gets in the way.
2. Long lines, no problem
3. Still a memory hog, but uses less RAM than Eclipse.
4. Easier to install than Aptana/Eclipse/Radrails
5. Support for in-IDE Mongrel execution, log display and debugging.
6. Most of the stuff you want in an IDE, if somewhat less than Eclipse. Most of the Rails specific functionality is presented when you right click the name of a project in the left hand navigation bar. Plugin installation, subversion integration, etc. lives here. I haven't tested any of this yet because I usually use the command line, but its there if you need it.
There was only one gotcha so far. The 1GB Linux machine was running Redhat ES 4, a two year old version. For some reason the fonts in Netbeans were screwed up on that machine, but not on my other Fedora 7 machine. So I switched machines and coded happily ever after.
You can download the ruby version of netbeans here.