I went from a 4B-ST (made at the end of 1999) to a 4B-SST2 (the latest one, with the new capacitors etc, made in Sept 2009). For information, the rest of my system consists of a BP-25, a BDA-1, and PSB Stratus Gold i speakers.
The SST2 is a much more robust amp -- music that pushes the ST into clipping doesn't even feel like work for the SST2 - it has tighter low end, and a clearer high end. It is not a super dramatic, day & night sort improvement -- the ST is a great amp, but the SST2 is both more refined and stronger -- a definite step up. I sold my ST, and that helped reduce the cost of the upgrade.
-- Ian.