Even without source maps it is not such a big deal as people who have no experience using cross-compilation would think. I write a very big game engine with GWT (http://www.webworks.dk/enginetest) and have no problems debugging. Clients send neat stack traces back to the server and I can even obfuscate the application code and using symbol maps deobfuscate the stack traces on the server.

I gave a presentation a couple of weeks ago with a couple of slides with example stack traces with and without deobfuscation: https://docs.google.com/presentation/d/1TCzHx1UH24gA1Au8LOvZ...