Cost: If you have access to metal work equipment then the total cost with all the fittings would be about $500. At least tripple that for machine shop work. I traded some other "hardware" to the machine shop for the fab work and I paid for all the off the shelf fittings, gaskets, and tanks.
Portability: Each conicle can be removed from the stand. They weigh less than a 6.5 gallon carboy and should be easy to clean. I am planning on making some stands for my spare refrigerator for lagering.
Capacity: I used the 6.4 gallon cones from Toledo Metal Spinning. I plan on filling them this weekend with an American Pale Ale and an experimental Red Ale.
Misc: I love watching the fermentation in a glass carboy, I was didn't want to mis out on some of the "show" with the conicals so the lid is made out of clear Lexan with a 3/4" NPT fitting for a blow off tube.
I can't wait to use them.