I recently bought this in navy blue. Haven't worn it a whole lot yet, but I just love the design: two of my favourite things - music and science - put together in a totally awesome way.
I think of the ones I have, my Initech-shirt is the nerdiest. But, the Retardi shirt's probably my favourit. I rarely use it though. Mostly at geek gatherings.
I'd like to have the "Stand back! I'm going to try science!"-shirt at http://store.xkcd.com/