Rubymine can, with one click, make an ER diagram of your ActiveRecord model -- from the code, not the db schema. That pays for itself right there, especially if you're learning someone else's code base. Try it for a day, you'll never go back.

I've used it before. I found it valuable, but there were a number of reasons I went back to a smart editor instead. I do use Jet Brain's DataGrip, so I'm a big fan of their tools overall.