I've just changed from Roto Trubass to Roto flats.
Only had the flats on for a few days so can't comment other than to say the G string feels like a razorblade. I'm hesitant to slide my finger along it for fear it may cut me. That said, it may just be in my mind as I've just come from the black nylons, which are much thicker.
Black nylons: lovely and warm, great for the plonk, plonk, plonk parts that I play. One thing I will say about them is that the B and E strings feel very plastic. Hard to describe but it feels like you're playing a solid (but flexible) plastic tube rather than a string. I use my thumb so it's not too bad but it felt 'wrong' playing with fingers; there's very little feel/feedback through them. Lower tension so they move about a bit more, especially near the neck. As stated, I'm a roots-and-fifths guy; I think more technical playing would prove slightly more difficult than usual with these. I had to file my nut to fit them, too and, because the strings are thicker, there's less gap between each string, meaning you have to be more precise with your fretting hand.
If you've any specific questions, just ask.
Oh, should it matter, the silk on these is gold. Great if your hardware's gold; not so great with brushed metal/chrome.