So, after using Gnome shell for like 3-4 days continuously... if you already feel like "have had enough of that shell!", then you might want to consider installing few of the below beautiful looking shell themes.
Well, if you're a bit of a Tron-Legacy geek and would like to make your Gnome Shell look exactly like Tron.... then you should install this theme from here and all the instructions concerning how to install the theme (including icons, etc) can be found in this deviantart page.
This is already a very very popular shell theme and I can see why!. The theme uses colors bit similar to the ones that are used in Mac OSX... and looks easy on your eyes as well. Get the complete pack from here.
Well according to the developer it is based on the Elementary OS theme and gotta say it looks really beautiful!. Although in the recent past the colors used to be a bit Greeny, but I prefer the darker Grey and the Blueish variations. You can get it from here (including the installing instructions, which are basically the same).
If you love deviantart and wanna make Gnome3 looks exactly like it... then install the deviantart theme for Gnome shell as well.