I'm using the UE Buffer Jack with the Cerberus III (custom, low-sensitivity). I usually play very soft piano-type songs to identify hiss, as it's easy to make out during the falloff between notes and pauses in between notes. After putting the buffer jack on, I couldn't hear any hiss - it might still be there, but it's certainly not audible.
I'm not noticing any loss in sound quality. Highs, of course, sound better now that the hiss is gone, and quiet songs, too, sound way better.
There seems to be a very slight reduction in bass. Almost indistinguishable, and within the realm of placebo, so I'm not sure about that.
In short, if hissing is a problem for you, this is probably the best solution. These will improve your highs by removing hiss, make quiet songs sound as intended, and have no noticeable otherwise impact on sound.
On another note, these are tiny enough that their size is negligible, making them perfect for travel. Another plus is that they add 3-4 inches to your cable.