I am using the default skin on both of my desktops, but on my laptop I use a darker-themed one. I just like how the default them flows - though, as long as it is FireFox, it is good enough for me.
I've tried a few other themes but I always end up going back to the default. I've had some issues with certain add-ons and themes. I like how clean the default theme is and like David I don't care that much about what it looks like as long as it is Firefox!