I agree with what you guys say. I pinched a friend's ODAC while we were in Tokyo for a while so I could give it a run. On sonic merit alone (ignoring everything else) the ODAC + O2 combo is great. The closest I've come across would be the Dragonfly or Audio-gd NFB-16 (not currently available) which are, respectively, a little flatter and a little-less-clear-but-more-musical sounding.
Some observations:
The ODAC isn't the first to use the output straight from the DA chip. The Metrum Octave does the same trick. The downside of this is that it depends on the amp used having a good input stage. Likewise, the ODAC sounds better with better power feeding it. The slight harshness in the sound disappears if I plug it into my Vaunix USB hub (designed for their lab-use signal generators). If I then feed the output into my Phoenix, which makes most DACs sound excellent, I could just about fool myself into thinking I'm listening to my main DAC + digital accessories. Other amps I've tried don't fare as well. The ODAC and O2 were designed for each other, so, not surprisingly, work perfectly together.
The O2 doesn't seem to deliver the dynamics of music as well as much more expensive amps, so the overall O2/ODAC combination sounds a little flat in comparison but does a lot better than quite a few more expensive amps. I'd rather listen with the combo than a lot of other gear I've had pass through here that didn't impress as much, but cost a lot more.