I have experience with Dan Collins' grips. One is a micarta imitation aged ivory, and the other is ivory. The micarta was a tight fit on a 1908 S&W triple lock, and needed care getting it on and off. Ultimately, I decided I did not like micarta, and swapped it, but the workmanship was excellent. The ivories are on 1928 S&W 3rd Model HE. These, too, were a tight fit, and I spent some time slightly enlarging one of the pin holes on one of the grips. It still is not a perfect fit, as I note a gap, ever-so-slight, between one grip panel and the frame. But my belief is that it is hard to get an aftermarket grip that is a perfect fit, and to do it right you need to let the grip guy have your gun to work with. I do know that in S&W's case, pre-war, grips were made for specific, individual guns. Judging by the close fit of grips/stocks to my prewar Colts, I think this must have been the case with Colts, too.
I believe the grip to gun fit gets easier with postwar guns, especially those built on modern CNC equipment, as they all come out the same size. Then again, in my case, I like the old ones better...
I think Dan Collins is a nice guy, and that he does excellent work.