I purchased a long sleeve shorty blacktec and got to test it's thermal capacity when my dry suit ripped in early spring. Here in Montana that means there's snow on the ground and the water is about 43-50 degrees. I had no problem at all. It was like magic. However, by the end of the very same season, it wasn't keeping me warm in 60 degree water and 60 degree air. There were so many pinholes in the suit that the original design was too compromised to perform properly. I didn't pamper it to the same degree that ToddL suggests, but I was always very conscious of being gentle with it.
I have since picked up another drysuit and prefer the extra warmth it provides during early and late season, but I will absolutely be purchasing another blacktec to replace my old one for next season. This time I'll employ some of Todd's hints.