I have Classic Auto Air. It blows cold! It frees up space under the hood. Etc...
The one negative I have, and supposedly they are changing this like Vintage did - is that you still use a cable to open/close your valve (for flow to the heater core). This seems to need adjusting now and then, and your control arms must be in good shape to pull it. I did hear they are going to a servo.
1968 Firebird 400 Convertible Original: Engine, Intake manifold, Heads, Body, Stereo, 3.08. THM400. Engine rebuilt by Ken Keefer early 2012. New: Holley 4bbl 670 HP, hidden iPod setup, RobbMC mini starter, A/C, front disc brakes