Up until today, I was coding Lilypond using the Lilypondtool plugin for jEdit. I just happened to find out about Elysium by googling on Eclipse and Lilypond. I installed it on my Macbook, and am quite pleased with it so far. I am running:
Macbook 2007 model
MAC OS X Snow Leopard
Eclipse Juno [v4.2]
I suspect, as was the case with me, many Lilypond users do not know about Elysium. It is not listed with all the other 'easy editors' in Lilypond's documentation:
Elysium, of course, is a lot more than an editor thanks to being integrated into Eclipse.