In 2011 I dove the Whites Diving MK3 Undergarment. It was the warmest undergarment I've ever worn, bulky, restrictive, and extremely buoyant. While I enjoyed the warmth, the other three qualities were less than desired. In 2012 I finally decided to upgrade to the Whites Diving Thermal Fusion to see if the hype was true. For the most part it was.
The undergarment was almost as warm as the MK3 and extremely flexible. I also went from having to use 32 pounds of lead to 20 pounds of lead on a single HP 100 setup. On my double HP 100's setup I went from 10 pounds of lead to zero during the summer and 10 pounds of lead to 5 during the winter.
I only have one complaint about the undergarment and that's the quality of construction. The stitching on the undergarment is very poor. After a few months of ownership the seams started to come apart and I've had to repair the seems multiple times.
I casually mentioned this to a Whites Diving representative at a dive show in 2013. They gave me their contact info and told me to contact them after the show and send it back. They said that the seams shouldn't be falling apart and that my unit was defective. After six ignored email attempts with zero response I finally gave up trying.