Well I have to chime in here as I recently put a TNT on my rig and had a simlar experience with CCM. They were out of stock on the cylinders when I initially called and over an 8 month period I called at least 6 times and heard the same story, out of stock and it would be a couple of months. I had heard of, and read, the horror stories of the OEM Kubota TNT setups but after the 8 months I had waited, I decided to go for the OEM kit. I had spoken to my dealer and relayed my concerns and was assured that if there were any problems with the OEM system, they would take care of them.
I have now had the dealer installed OEM TNT kit for 6 months and my results may be out of the norm but I love my setup. As far as I can tell, it does not leak down and this is after 2-3 hours of box blade work and 4-5 hours of mowing.
I let the dealer install my set-up and while I probably ended up paying too much, I have been using my setup free from problems for 6 months now. Your mileage may certainly vary but I am completeky happy with my OEM TNT.