Like Foxgap, I haven't given them a fair enough shake and intend to this year. Geebee, they'd be less a problem than standard mono leaders. Not as stiff, less spray. Fish don't spook by seeing leaders/tippets, so long as it's floating naturally. Stuff floats down the stream all the time that doesn't bother them.
My biggest hangup was the customizability thing that Swattie brought up. I'm constantly rebuilding leaders and see it as an advantage. Not just tippet. We're talking removing mid sections to shorten them, adding sections, etc. I often use anything from a 5' to a 12' leader, and everything in between, in the same day, WITHOUT changing leaders.
But perhaps if I just got a bunch of them and was willing to change leaders all day long. Or have some section system like Swattie was talking about.
I have no doubt they turn over better while still being more supple, and thus would be great dry fly leaders. Also, stretch is important for big fish, but that's mostly nymphing and tossing streamers, where I'd likely not use them. For dries stretch isn't usually so important.