I was an original user of AirPak - It is pretty easy to use... I've modeled things ranging from bank vaults in a fire senario to large airport spaces with solar loading natural and forced ventilation...
The interface is designed to work you though model creation (assuming the user doesn't have...
I am a user of both CFX & FLUENT...
At this stage I prefer FLUENT over CFX - although I will say the CFX developement team is very responsive to improvement requests.
I find the UI for FLUENT to be less confusing... I find Fluent's pre-processing and solving steps to be better... However...