CrimsonKing said "Have bought every pair I've owned there. Cheapest I've come across, you get them quick, the ladies are great. I've never tried zenni but really want to. Who can tell me how to measure their pupillary distance properly? "
They have a tutorial right on their website, with pictures.
I did mine by their instructions and have ordered a dozen sets of glasses with never an issue - so I must have got it right.
I just ordered 7 pairs for myself and my daughter. I have only had one pair turn out to be not the best fit (too big for my face) but they have all the dimensions and measurements to help you.