How to use EM's when you're already comfortable with pixels
Here's why I use em:
Pixels are precise, but they're not very flexible. If you increase your browser's font-size, things may grow in wacky ways.
Percentages solve the issue of flexibility, but sometimes the math gets tricky, and I don't feel very comfortable purely working in that mode. I believe in Rapid Development, which in this context means fewer calculators.
By translating 1em to equal 10px, I can think in terms of Photoshop pixels but also ensure that increasing the font-size will allow the site to grow proportionally. If you know how to multiply and divide by 10, you'll rarely need a calculator.
Note: The only time I do use px is for 1px borders. 0.1em doesn't work very well.