googling in this forum for a conversation about two-and-a-half years ago ought to turn up an extended discussion. Or, check in the M&P 340 or the 642 threads, using the same search technique.
I have them--and if you train with them, they work. However, there are two strong disadvantages--one, they are just about 'un-carryable', at least bare--the cartridges easily come out of them, just because of the pliable nature of the material, as in the founds falling off the loader in your pocket.
Perhaps more importantly, at least one top pistolsmith (Grant Cunningham, IIRC) recommends NOT using them, for that later swipe can, he says, bend the yoke / crane, or the extractor rod.
For my personal use, I have NOT replaced them in daily carry for my M&P 340 (j-frame, five rounds). One does sit in a console pocket in the car--but, as my carry habits have evolved, I have just concluded speedloader reloading is NOT a viable tactical move for me. When I worry about that need, I carry a semiauto--for example, a P3AT c/w two magazines.
Jim H.