I learned to circular breathe a few years ago, but with almost never any need to do it, I'm still not particularly good at it. I learned more or less by trial and error after I learned the basic principle of how it was done. I impress my non musical friends and younger undergraduates with my water-spurting-while-breathing-in demonstration. I guess the point is that for me it is simply a parlor trick.
I can do it on a sustained note, and even then on only certain registers of the instrument that provide the right level of resistance, usually only in the tenor register. Forget circular breathing on low D or something.... What I'm impressed by, and I assume this only comes with practice, is circular breathing while changing notes like in a technical passage (ala the performance of Flight of the Bumblebee that I'm sure we're all familiar with thanks to YouTube. - wink wink, Mr. Ewell.).
I will definitely take a look at those videos when I have a little more time and can mess around with a bassoon in my hands.
M.M.A., D.M.A. University of Illinois at Urbana/Champaign: B.Mus. Lawrence University
Bassoon professor at University of Wisconsin Eau-Claire
Maker of the Little-Jake electric bassoon pickup and Weasel bassoon reeds