I was originally planning to ask if there was an easy GUI-friendly way of installing new themes on GNOME/Unity, but ...
I've been reading some blogs recently that seem to indicate that GNOME and Unity are meant to not be customizable: (e.g. this thread on Hacker News and the associated article).
Is this true? I'm trying to ascertain if its worth time to customize Ubuntu GUIs, whether in GNOME or Unity.
The major issue the author has is one that developers and cutting edge users have to face. Don't get me wrong, I don't enjoy the Gnome team breaking the API with every point release. However, that doesn't mean that Gnome and Unity can't be customized, nor does it mean that it's infeasible for the average user to do so. While on Ubuntu, your version of Gnome and Unity are going to remain stable (there is a very, very short list of applications that get major updates in the middle of release cycles). So worst case scenario, your customizations would need tweaked when you do a release upgrade (ie - go from 12.04 to 12.10), and even then, any extensions you use will have updates by that time.
Both Gnome and Unity are still very much customizable, and both have GUIs. Ubuntu-Tweak for Unity (and some of the underlying stack) and Gnome-Tweak-Tool for the underlying Gnome stack (the GTK theme, etc). (Fair warning: there is some overlap with the two as of 12.10, due to some things that Ubuntu-Tweak has removed for some reason, so be careful with where you change the overlapping items. IE - GTK themes can be controlled in GTT, while the shell theme needs to be changed in UT.)
You can also use CCSM for things like wobbly windows, if you want them. However, Unity does set up a bunch of its own settings, so you may have to deal with conflict resolution to change Compiz stuff dramatically.
Additionally, Unity can be extended with lenses, which can add a ton of functionality (I absolutely love the calculator lens).
In short, it depends in part on what customizations you're looking to make. For colors, sizes, and certain parts, you can pretty much customize to your heart's content, but for some things, such as the location of the top and side bars, there's not as much available.