I bought my first set of flat wounds for the Rick bass last night and remembered this thread. Here are my initial observations from a first time flat wounder. I bought Roto Sound 77's, 45-105. The tone from the strings is heavier, has more punch and a more even tone from string to string. The feel of sliding up or down to a note is really nice. They have more tension on them than the round wounds; my hands were not used to this extra tension and got tired more quickly. I am going to have to adjust for intonation if I leave them on. They seem to be sitting up just slightly higher off my fret board than the rounds; not much, but the extra tension must change the neck slightly. I can try lowering the bridge slightly, but it may require a slight neck torque.
Overall, I like the tone a lot. Think McCartney sound here, really nice bass tone. I am just not sure how my hands are going to handle the extra tension; I had some tendinitis issues with heavier guitar strings a few years back. It's not like I gig out every weekend anymore, however. I read up on flats quite a bit last night after I played them for an hour. Most every comment said to give them a little break in time. Sounds like there may be some brands that naturally have lower tension as well.
All of you flat wounders out there, what brand strings are you playing?