Hello,
Great thread!
I've been fermenting in Cornies ever since I dropped a carboy and watched a lovely looking porter flow down the drain in our wet room (luckily it wasn't in the kitchen, or I might not be here to post...), and think they're great!
No problems with attenuation, just make sure I pitch plenty of yeast, and I do give a little shake every now and then, more to do with poor seals and little air lock activity.
Dry hopping is a piece of cake as you can hang a muslin bag in from the nice big opening.
I haven't tried transferring under CO2, as I bottle everything, but this looks like the was to go.
I split 5 or 6 gallon batches between two cornies at the moment, but think I will go for 5 gallons in one and try to top crop (Chis Whites & Jamil Zainasheffs book suggests this is the best time to harvest), there was a simple design in the gadgets issue of Zymurgy, and this guy has a good idea for the evil glass:
http://www.aleiens.com/video/top-cropping-yeast-from-a
One thing which springs to mind is that it's probably really important where you take the blow off from? If you take it from the long dip tube (outlet?), then is there not a chance a siphon could be started with the yeast to start with and then the hard won beer, totally emptying the keg? Or, the pressure bulids up above the beer and then pushes it up through the dip tube, again starting a siphon, bit like transfering? I would think having the dip pipe adjustable for harvesting, and certainly above the level of the beer to ensure it can't disappear, unless pressure builds up for some reason is the way to go?
Cheers & sorry for such a long post!
Rob