I've got Powerbass, Jazz Flats, and Jazz Rounds on various basses, and while I like them all for different things, I have really fallen for the Jazz Rounds as my main string. I've put them on my MIA 75 Jazz Reissue, my US Curbow 6 string fretless, and my Curbow 8 string fretless (bottom strings only).
They are absolutely effortless to play, and sound better than any other string to my ears. plenty of usable tone. Nice and full. I also find that they respond better to different right hand positioning, too.
If you want to hear them, here's a scratch mp3 of my new trio:
http://www.mindspring.com/~jshew2/FreakoutPillow.mp3
That's the Jazz with JRetro/Fralin pickups with Jazz Rounds. I really love the way they sit in the mix.
The smaller windings and lower tension make them the perfect match with Ken Smith piccolo strings on the fretless 8.
They even sound pretty good when you slap them (admittedly not as good as the Powerbass, though).