This revival is inspiring me to write a bit of my own review:
In the month and a half that I've had the tailpiece, I've found that the most noticeable change it made on my instrument is an increased snappiness. Right now I've got a set of Spiro mittels on, and the tailpiece really gives them an almost gutlike elasticity. Of course, they're still somewhat stiffer, and they still sound like Spiros. It's really made my already quite playable instrument very, very easy to play.
Tonewise, they've brightened up my instrument quite a bit, and I perceive an increase in sustain. Everything's feeling and sounding looser and brighter. Double and triple stops really ring out now. Not really for everyone, but it really suits the sound I'm going for.
Most of my observations come from a jazz standpoint, as my classical arco experience is super limited. I do, however, sense that the strings have become much easier to start, and whenever I'm practicing or soloing with the bow, there are more clear notes than before, particularly in the middle and extreme high registers.
Also, I use a quiver with mine, and I've got both of my pickup jacks mounted on it. I avoided the quiver for a while, but I actually felt like, on my bass, the tailpiece would benefit from a little extra weight. Without the quiver, open strings had a tendency to sound hollow, with perhaps some of the over resonance that Eli described. Putting the quiver on solved any of those problems. Now the tailpiece is working great.