I also like the Big Dot that I have on my 340M&P.
I DO NOT like the ramp front sight that came on my 640. My eyes are getting a little old and I can not focus on it.
I'm experimenting with bold paints for the front sight. I just can't decide yet. It seems that a bright white is about as good as I can do for quick pickup.
Interesting to me - is that Jerry Miculek of Smith and Wesson revolver shooting fame uses the fiber optic sights for all of his cometition guns.
He alternates between the green and the red for different events he competes in. Seems to me that you have to think that if there was something that was better he would use that wouldn't you?
They are not only bright. But they are pretty big "dots" out there as well.
I may look into them. Perhaps I'll just remove the ramp that came on the 640 and go with one of those.