Mozilla 1.1b is definitely improved over 1.0. I've had minimal rendering issues and stability has been top-notch. There are a few little things here and there, but I've found myself using Mozilla as my primary browser.
As far as themes go, I like the standard modern one. It's clean and efficient.
I do have a question, though. I couldn't find this in the options anywhere, but I want to know if there's a way to set up the browser where, when I clicked on links that they would open in a new tab instead of in a new window. Having to right-click on links can be a pain in the butt.