Been using my cornys for secondarys for some time now. Even when using as a secondary, I transfer (after ditching first pour or so with sediments) into another keg with priming solution to either leave there or use it as a bottling bucket to fill bottles...I try to use kegs for everything and get much cleaner results transfering...make a jumper from keg to keg, etc. No siphoning or anything...even bottling is easy with pressure.
I've not used cornys for primary because I'm afraid of the batch blowing up with the lack of headspace and potential to loose lots of liquid. I tend to use 6 or 7 gallon glass with an airlock, no blow off tube (again, it hurts me to see all that foam turn to liquid and be wasted)...I use glass, particularly for the primary because I like to see what's happening. My two cents.