I also have these coilovers. Been installed on my Z34 for a little over a year now and have been nothing but excellent. As a Z32 and Z33 owner as well, I have also owned there coilovers for several years.
They make 3 different versions of coilovers:
Suspension
I have been running the Ultra-lites on my Z32 since 2003 as it sees a lot of autocross and road coarse and I run the SS on my Z33 (since 2006) and Z34. The SS is specificly designed for every day street use with just enough performance for the weekend track warrior. But if you are more serious on the track, I would recommend the Race or Ultra-lite coilovers. Both Race or Ultra-lite coilovers are still more than comfortable on the street but just have more stiffer damper settings and pillow ball mounts.
As for back ground, Powertrix has been making coilovers since 2002 or at least thats when Charles intorduced the Z32 line to us and the San Diego Z32 club meeting. If they look familiar to other coilovers on the market, its because they do manufacture for other companies for rebadging. However, they are in no way affilated with BC coilovers.
For the Z34, the spring rates are 12k/10k on the SS and 13k/11k for the Race and Ultra-lite.
I'll post up some pictures as soon as I figure out how to do so.