It really depends on the purpose of your design. If the site is actually primarily for mobile and you're just including desktop as a courtesy, then Mobile First makes sense.

For me it's kind of like working with a jpg image. If someone gives you a full-size 300dpi image, you can always reduce it and not lose much quality. If they give you a small one and you need it large, you can't enlarge it without some serious pixelization. It's much easier for me to start with the large design and remove or change elements as the screen becomes smaller than to expand them going the other direction.

Mobile Layout is hard. Because you have to think small then large. When we are all used to thinking large and then small.

It really does not matter which you decided to use as in the end you will have the same product. However, it is a lot harder (even for me) to do mobile first. I like to see a website in full and then pick and choose what I want my mobile users to see. Everyone is different and codes differently so choose a style you like and run with it. You may find that you like mobile first better as it allows you to hone the content you are using.

Adding on to Adam's comment, I too do the desktop view first, and one of the other reasons it works better for me this way is because much of the "mobile" settings are already built into the program so things like your menu and images etc. will already adapt for the most part with a few alterations along the way. I don't think it's that user friendly going from mobile to desktop direction

Thanks all, great thread!