I've putted them on my PRS Kestrel bass with single coil pickups.
Feeling is great, so different in compare to regular roundwound strings, They are smooth, movement of fingers is silent. Very interesting experience.
About the sound, they are warm and in compare to roundwounds, even dark. They have a great fundamental tone and power. For me it is OK, but I suspect that not every one will like it.
Slap and pop technique sounds good, but with flats you will lost this "zing" and percussive attack. Slap on flats is more like background playing.
Another thing is that flat strings keeps their sound for very long time.
IMO every bass player should try flats to see where is a difference.