I had a Jackson with one installed, and found that they also improve the clarity and strength of the guitars tone. I didn't realize how much of a difference it made until I was changing strings one day. While the string tension
was loose, and therefore the tremsetter was not engaged, the tone sounded normal. Then while tightening one of the strings, I suddenly heard the guitars tone get quite noticeably stronger and clearer. Curious, I detuned the string and heard the tone go back to normal. After a bit of experimentation, I noticed that the tone got better right at the instant that the string tension became strong enough to set the tremsetter in it's engaged position.
The big negative to the tremsetter is that it does make the trem a tiny bit stiffer, and pretty much eliminates flutter. Since I don't use trem flutters in my playing, and like the greater degree of control that the stiffness provides, I think that they are a good mod.