I've bought these strings for a custom-made 6 string fretless bass. I mostly use this bass for recordings and practice, since I play fretted basses when gigging. Therefore, my experience with these strings stems mostly from studio work, and a single, specific instrument (with active EMGs btw).
That said, I was really impressed with both sound and playability; in fact, when I went I had the bass professionally set-up, the workshop guy commented on how nice these strings sound, especially when it comes to getting the "mwah" sound (the fretless "holy grail"). They are also very clear (i.e. not muffled at all), which means that there is no need contemplating the use of roundwounds anymore.
Most importantly, the strings are so easy to play that I am even considering trying an even heavier gauge next time (though not really sure about this); it only took me about an hour or so to get used to them, and it had been quite some time since I've last played flatwounds. By the way, I've been using the strings for about one month and a half, with no change in the sound or feel.
I would also like to praise LaBella's customer service. I contacted them because I had a minor issue with the C string (not related to its quality, but its length); though we were not really able to pinpoint what the problem might have been, they send me a replacement string from NY without any hassle at all.
In a nutshell, I believe that you cannot go wrong with these strings, at least on a fretless - they are exactly what you'd hope for.
