How effective are the under dash AC duct outlets on a original car?

The problem with putting vents in the dash is, there's no room to run the duct hoses back there! It's tough enough just trying to snake the defrost ducts into place- imagine trying to fit in another 2-3 hoses. The factory underdash vents work pretty good, they're fairly wide and the individual vents can be directed to distribute the air pretty well; the dealer installed and aftermarket knee-knockers are pretty localized, but they are what they are.