I've always liked this saying and its presentation. It's too bad that it is now on everything - from coasters to mugs to blankets. There is even a variation - "Keep Calm and Eat A Cupcake". Yeah, not quite the same.
I remember I first saw this poster at the FDCC years ago in what was the most stressful meeting in my life, to date. It made me take a deep breath and continue. I then proceeded to spill water on the president of the FDCC. Twice. I was made to go sit at another table.