Like JamesG I hope you don't mind my presence here- I am a low hours PPL, but have a few hundred hours gliding and found this thread quite interesting.
I spend quite a bit of time in gliders each summer inside towering cu's and the like (yes intentionally and for fun!) soaring the updrafts sometimes to quite suprising heights. The effects JamesG describes, to me sound perfectly indicitive of a powerfull convective updraft. In a glider, as I'm sure many of you know, the ideal when soaring in cloud is to circle in the core of the updraft. In my experience this core is usually exeptionally smooth, (and sometimes exeptionally strong!) It all gets a bit more interesting if your circle strays from the core. The boundary between the rapidly rising and equally rapidly descending airmasses can be quite turbulent to say the least. Holding a steady attitude, the airspeed and vertical speed fluctations as you cross this area of shear can be pretty amazing. Usually it's just a case of riding it out with steady, smooth IF. In a glider it's no big deal because you are flying WITH the system of the cell, not trying to fight your way through it at a nice steady speed and height as in a powered aircraft.
I would imagine that unitentionally encountering such conditions in a powered aircraft would be a whole lot less enjoyable!!
That's my perspective on it anyway!