To me, they quickly settle into the sound of dead rounds, but with less finger noise and a smoother feel (the stickiness goes away). They stay that way for a long time. If rounds are too zingy in your mix, but flats are too dark, they might be just ticket. Turning down the tone knob can muffle the fret clank and right-hand attack or articulation, especially for slap or pick techniques.
My fretless was wearing well-seasoned LaBella flats - the darkest strings I've tried. With a new group, I wanted a little more presence, without getting anywhere near the Jaco sound. I had a spare set of Half Rounds in the drawer, and they're doing the trick for now.
They're definitely not for everyone.