There is an almost identical thread started by the OP, only without the poll. Same thing: the limiting factors in using a 223 for deer are bullet construction and shot placement. Use a good bullet, and it will perform adequately. I have known people who regularly use 22-caliber centerfires for deer, although I do not. If you read all of the posts, you will notice that most of the people who claim they are using 223 for deer are not using 55 gr varmint bullets.
I think that with proper bullets, a 223 can cleanly harvest a deer in many places around the country. I say many places because there are areas where long shots in windy conditions are the norm. There are also areas where the deer run up to 300 lbs on the hoof as opposed to the areas where the average deer runs 80 lbs on the hoof. When you combine large animals with long shots in windy conditions, there is less margin for error, and I think the 223 is a poor choice in those cases.