Aslak, I've tried the extension on quite big project and it looks like jrebel reloads all classes from exploded directory even if they don't change (I didn't modify sources). It's probably because create date changed for the files.
For big project this results in poor performance.
Also speaking about performance, it takes lots of time to explode medium sized project.
They do change, since you export the same files over and over. JRebel is based on timestamps of the files in the exploded dir. This is where the file reference to the souce file on disk comes in.
The default ExplodedExporter in shrinkwrap is single threaded, maybe a multi threaded exporter would help?