When to use gconf vs dconf?

dconf is a new way for applications to store settings, and it is intended to replace gconf. dconf-editor and gconf-editor are for their respective configuration systems, and they don't talk to each other. At the moment that transition is still a work in progress, so many applications continue to use gconf. In addition, some applications still have settings left over in gconf even though they are using dconf now.

Almost all of the software that is installed by default - everything that comes from GNOME - uses dconf (if anything). Almost everything else that is available uses gconf, though this is of course changing as older software is brought up to date.

So, yes, changing a setting in gconf is likely to do something! If you did a fresh install of Ubuntu 11.10 and didn't migrate your gconf settings, it's likely that everything you see with gconf-editor is being looked at and understood by some current application. If your settings predate 11.10, there may be stuff there that isn't being used.

As a rule of thumb, check dconf first and if what you're looking for isn't there, try gconf.

Not all applications have been upgraded to use DConf, but still use GConf. Those apps must be configured using GConf.