Yep, couldn't agree more. Never tried flats personally, but I found nickel roundwounds to (usually) be lacking in mids when they're brand new. Once they're broken in a bit, they sound great - very balanced, with enough treble to accentuate aggressive playing, but not so much that string noise is an issue. But after 3-4 months, they start to lose ALL their top end, and the low end doesn't actually get accentuated. The result is a fairly thin sound, regardless of how you dig in.
However, I'be been using Rotosound Swing 66 strings for two and a half years now, and they're GREAT. They're as durable as my old D'Addarios (which was one of the big pluses about those), but I think they age better. They are a bit brighter when they're new, and a bit harder on your fingers, but I think they age better than any other nickel roundwound string out there. That's good, because I don't like changing strings if I can help it (partly for economical reasons, partly because I have my strings on the low side so lots of zing kills note definition). One of my basses has had the same set on for over a year, and the other's had the same set on for 6 months; both have plenty of low-end, but with plenty of upper-mid range definition. I've heard that they chew up frets faster than most nickel RWs, but I think that isn't a problem if you leave them on for awhile.
I'm really happy with them because I've tried many brands, with the particular focus on finding a set that I can leave on for over a year without it just sounding DEAD. Fender strings broke too easily, as did Dunlops (though that was a few years ago, and I've been told Dunlop have really lifted their game since then). Ernie Ball's were good, but a bit expensive (I've never even tried DR strings just because of how expensive they are), and didn't come in the exact gauge I wanted. D'Addarios also didn't come in the exact gauge I wanted, as didn't Warwick strings (they also didn't age so well). In fact, the only mass-production companies that DO make my exact gauges are Dunlop, Rotosound, and DR.