For a really fast Rails environment I highly recommend Zeus https://github.com/burke/zeus which is built in Go. It does a great job of reloading only the dependencies that have changed.
Agreed, although improving load times all across the board is surprisingly helpful. It can make restarting your app in production after a deploy faster, for instance. If anything it just makes you happier in dev if you're running some Ruby command outside of Spork or Zeus.
I'm not sure if I fully understood your comment - You've used Zeus in production?
If in reference to just improving speeds all across the board - recovery after deploy is the #1 priority for me. Rolling restarts feel like unnecessary complexity in deploys.
