Eagle 6.x installed to program files\eagle 6.x\
Eagle 7.x installs to c:\eagle 7.x\
I have 6.4, 6.5, 6.6, 7 and 7.1 installed on my system as long as you change your home directory and not overwrite defaults when first run, your files and settings will stay the same.
In answer to your question though, UI wise, different icons...pretty much it.
Hierarchy addition, if you've done without it so far...
Autorouter upgrade to use more than one core, if you autoroute large boards then it might be worth it, otherwise, probably not.
So summarise, if you don't care for icons, hierarchy support or the autorouter, then no, it's not worth the upgrade.
HTH!