I've just changed the originally supplied strings on my Modern Player Fender Jazz to these strings, and to my taste a substantial improvement, as I like a deep mellow tone.
Flatwounds traditionally have the reputation of having less sustain, but I haven't found that, although it could be that the old strings had gradually deadened as they gunged up.
I've usually used Rotosound flats in the past on both fretted and unfretted basses, and I prefer the sound that I get from the Fenders, although the only direct comparison I can make is with my Maya fretless with old Rotosound nylonwrapped. For what it's worth, I have Fender nylon wrapped strings on my acoustic bass which although not designed for acoustics, do work, but aren't totally flat and so generate fret noise. The nylon Fenders don't sound as good as these stainless ones, but that could be down to magnetic versus piezo pickups.
On a general note, I've always found that flatwounds last much much longer than wirewounds, aand are kinder to frets and to fretboards (especially fretless fretboards).
I can thoroughly recommend these strings as long as you are not looking for a 'cutting through' bright tone
