I've seen a deer crouch down, turn 180 degrees, and get an arrow in the head in the time it took an arrow to travel 35-40 yards (a buddy of mine was lucky enough to have had his dad taping his hunt when this happened to him). After seeing this, I'm convinced that my bow isn't fast enough to take a shot over 30 yards, no matter how good my shooting ability is. Too much can happen during the time it takes an arrow to travel that far to make me comfortable with taking a longer shot. That's just me though. There are others out there (with faster bows, perhaps?) that can, and do, make longer shots connect.

It comes down to what you're comfortable with. My personal rule is, if I'm hesitant to take the shot, I don't. I bent this rule a few weeks ago and paid the price (just nicked a buck that was moving...first and last time I took a shot at a moving deer). That incident reinforced my belief in following that rule.