Cadsoft Eagle 5
installs to dedicated version 5 directory -- the new version won't overwrite a previous install. You still have full access to your original stuff if anything goes wrong.
When I start Eagle 5 it attempts network access; this might be an auto updater or license checker. It didn't complain when I permanently blocked access.
All my old schematics and circuit board files from Eagle 4.16r2 load in the new version. The board layout editor got a minor makeover with "web 2.0" pastel color shades.
Say goodbye to the awkward ALT+Backspace undo, because Cadsoft joined the rest of the world and standardized on CTRL+Z!!! Need I say more? Oh happy day!
Eagle now has right-click context menus in the schematic editor and the board layout editor. While context menus are ubiquitous (does your browser have it?), previous versions of Eagle pretend the right mouse button doesn't exist.