That's interesting that B&K products aren't very much discussed. Anyhow, I purchased the B&K ST125.2 about 3 months ago so I'll try to give you my impressions of it.
I have the B&K ST125.2 driving JMLabs Cobalt 816 speakers. Associated equipment is a Rega Planet 2k and a Conrad Johnson PV-10BL pre-amp. I originally had a NAD C350 driving these speakers, so my main point of reference amp-wise is the NAD.
I'm not good at writing reviews, so I'll just come out and say that I think that the B&K is a great amp. Compared to my old NAD its sound has more weight, authority, and power, but at the same time is a lot more relaxed. The sound is very open, with good space around the instruments, imaging, and a wide soundstage. The harmonics have a very natural sound to them, i.e. when vocalists sing together or a string section... they blend more naturally instead of sounding edgy or strident. I believe this can be attributed to the MOSFET design of the amp. The bass isn't ultimately the tightest or fastest that I've ever heard(although to be fair, the limitation may be in my speakers, I don't really know. They're -3db at 43hz) but it is deep and full and never runs out of power. I was listening to a recent Telarc issue of Beethovens 9th last night with the volume at an obscene level and there was no compression or confusion at even the loudest passages. It's a relaxed, pleasant sound, but it has the power and grunt to get down when you need it to. I listen to every style of music, classical, jazz, metal, rap, etc, and it performs everything well. Again, comparing it to the NAD, the sound is very full, open, and relaxed while at the same time sounding way more in control.
So while I can't think of a real weakness of this amp, I might hesitate to pair it with speakers that are too laid back, or in a system that's too laid back. While the detail is there, it doesn't smack you over the head. It's also probably not the last word in dynamics or in transient snap. My speakers tend towards the bright/lean side, and I think this amp helped rein them in a little bit. Not that it gives a colored presentation, per se, just that it will probably match better with some gear than others.
All in all I'm very happy with this amp. The sound and build quality(its built like a tank, with great fit and finish) that you're getting for $800 make it a steal, IMO. I'm sure you can do better, but I don't see it happening at anything near this price point. Sorry for the length of this posting, but I hope that it helps.