Edit-compile-test-edit-compile-test. This is the "software development cycle" all programmers know well, from "Hello World" onwards. JavaRebel is a JAR file which will allow you to skip directly from "edit" to "test" while eliminating "compile" (most of the time).
Using JavaRebel is extremely simple: just pass an appropriate command when you invoke your Java Virtual Machine. It took us about 30 seconds to get it working in Eclipse. Once it's there, it's transparent - and useful.
To test JavaRebel, we launched an application, then, while the application was running, added in some additional output code to the event handler for a button.
After a second or two, we received a notice in our console window that the relevant classes had been reloaded, and the button now executed its modified behaviour.
We can foresee this saving a tremendous amount of debugging time. Even a few minutes a day saved re-launching apps adds up, over a year, to hours or even days of productivity, depending on re-deployment time after minor edits.
There are a few changes it can't handle - you can't change class hierarchy or implement new interfaces, for example, but it's unlikely you'd be making changes like that during a standard edit-compile-test cycle.
There is also a risk factor; if the app you're working on is "live" and you are careless with your configuration, you could introduce new bugs into running code. However, that's a user error and hardly the fault of the program.
The trial version of JavaRebel lasts for 30 days and prints a message in the console window when run. This should be long enough to determine if the utility provided is worth it.