Cookes are my main collecting focus. Yep, they are definately great pipes! Other than getting a great smoke, you get a pipe that is uniquely Cooke in both shape and style. His blasts are the best, but I am partial to his smooths, which are now becoming very difficult to find.
The air channel is 3.5mm, which does not, IMO, effect the smoking quality. I do have one early production Cooke (Levin days) that must have a restricted airway somewhere in the stem...it whistles when blown though. Still smokes good though.
If you have the opportunity (act FAST), and the dough (700 plus), pick one up. Ebay is the place if you want one relatively quickly, but I often see them go for more than an estate pipe should go for. Supply and demand...
Stay away from his early works...Cooke himself will tell you that. His later works 2000 and up, is the finest example of his work.