The gains are pretty much summed up on the homepage
, so I would read there. However:
1. The ability to use and browse folders, the ability to disable SD card scanning, menu customisation, custom fonts, and a near infinite amount (well, it seems that way) of other customisation possibilities.
2. No idea, I've never had anything go wrong.
3. Worth it? It's free and hugely expands the possibilities of the reader. How could it not be worth it?
4. No idea.