They really lend a much warmer, smoother sound. Definitely sacrifices a little of the detail up top, too, but I'm generally ok with that--less fatiguing. They don't rock as hard, but they're much more comfortable and easier to use for long listening sessions. I still find congestion to be a problem with the HF-1, and the flats make that a bit more pronounced. Overall, though, the flats are a big improvement to my ears.
I guess that wasn't all I had to say about that.