More often when I'm leaving the house very early in the morning (ie. 6AM or whereabouts), I leave the A/C off for about halfway to my destination or until I start to feel a little discomfort. I try to save up some gas, at least during this leg of my daily commute. It all adds up eventually.

At idle, my car uses about 40-50% more fuel while the compressor is engaged. Though a bit OT, even leaving the shifter on Drive, Reverse, or any gear other than Park or Neutral (A/T only), will use about 20-30% more fuel when idle. I tested this on my 2007 Civic FD 1.8S A/T.

I have a video about the test which I made. Unfortunately, I can only show the link when my post count reaches 10 or above. I'll update this thread then.