I too have had 4 sets of coilovers on my TT Supra's. I am very meticulous when it comes to suspension. So I speak from experience, not just hear say. Although my experience only relates to driving 'spirited' drives on the street.
I have had: Tein HA, Tein SS, Tein Flex and now I have the HKS RS. The HKS are hands down better than any of the three Tein kits in my opinion. The Flex were a good coilover, however the build quality of them is not up to par with the HKS'. All of the Teins began to making noises which pertain to the pillowball mount design so they made more noise in the cabin. The HKS RS are much quieter inside the car. The HKS RS coilovers were R&D on the race track as well as the street for over a year in Japan. They were specifically designed for the US streets which are usually in much worse condition then in Japan. For around $1600 they are the best money in my opinion for a street car. However if you track your car (road race etc) then you would want to get a coilover with a higher spring rates than the RS.
This is just my $.02 from experience.
Chris