I've used it for ages though I did get a lot of ribbing at work when I was projecting during meetings, Right now it isn't available for the Firefox 4 beta so I'm trying the beta out on only one machine.
I did have the blue cats one installed (I like blue better then green), but for some reason the cat part got all funky looking so I switched to the green.
FF has some really cool themes to choose from. I thought about trying some of the others (especially the GB Packers one) but just can't change it because I love the cat one so much!