IMHO Ruby is a great choice for cross-platform GUIs. However, it depends on what GUI toolkit you use. The tools are: Qt, Tk, wxRuby, Shoes, GTK etc. I chose GTK because you can use the Glade interface designer to build your forms. You can see several examples of GUIs build with ruby at:
http://www.visualruby.net
About the programs' appearance, I've tried my programs on Ubuntu, Win7 and Win XP, and everything looks perfect. Also, if you look at Tks website, they have screenshots of TK GUIs in various platforms, and they look fantastic too.
The major benefit to writing GUIs in ruby, is that you get to use ruby instead of java. Personally, I find ruby a joy to work with. Good Luck.