As my strings go dead quick enough, I haven't used flats in a LONG time.
However, on the roundwound front, I've recently tried the Dean Markley 'Rocco Prestia' set, the NPS Roundcore/Nickel roundwounds and am very pleased with them. I was intrigued to try a round core set (thinking the wrap to core contact had to be better with no shoulders on the core as with hex cores) and I always prefer nickel to stainless (which are too bright for me till they lose that new string edge).
Tension wise they fall between Rotos/Boomers and DRs/T-Is. Loose enough for finger bends, but still a good tightness in the E and B. Strangely enough, when I first installed them, I did the stretch/pull to get the new string looseness out, and they never crept out again. Interesting.
I'd wanted to try the Will Lees, but didn't like the contact core on the low strings. I'm getting excited about these as so far they've been very consistent. Some strings just sound different on different days to me, and these have stayed the same tonally from day one. And Markleys pass my other requirement: About the same price range as most strings and available most places cheap!
J o e y