What benefits do JS apps have over a server-side language such as Java, C# or Ruby? Especially in an enterprise environment.

In most of my limited experience JS has been harder to work with than almost anything I have encountered. My personal experience using JS client side apps has left me feeling sour. Every time I visit a webpage and immediately hit a JS loading screen I cringe. Gmail is about the only I dont completely dislike that I know is written in JS.