Oh well. I understand the Full Attack option.
Compared to the other actions that are allowed: Charge, BullRush, Disarm, Trip which can be used effectively in any combat but seem balanced. AoO to balance it out or a -2 to AC with a +2 to hit. These are good tradeoffs. A CDG allows a 5 foot step, a guaranteed hit and double damage with an extreme fort save against one AoO for each creature within 5 feet.
In my opinion CDG works well outside of a normal combat round. I have agreed to the point of being blue in the face about a approaching a held of sleeping person while you are unthreatened, but in a normal combat with multiple attackers and lots of things happening a CDG just seems overpowered. A guaranteed hit (at any level for any creature) does not seem to be concurrent with the d20 system. This is a game of chance, in any case, shouldn't they have to roll? How many times have you been fighting something that is as big as a barn right in front of you with an AC less than your attack bonus and you've rolled a one and missed?