Not much, but I'll share what I believe to be true. I have shot one but not extensively and have no long-term experience. Bill Laughridge of Cylinder and Slide sells the parts to convert a conventional single-action Hi Power to the "Safety Fast System". For information check the following link:
(If the link is no longer valid, contact Mr. Bill Laughridge of Cylinder & Slide.)
It can be installed and used or removed and with the original parts back in place, the gun functions as it did before, i.e., single-action.
Hi Powers can also be purchased through the maker Fabrique Nationale (FN) in the SFS form. FN manufactures the Browning Hi Power. Browning Arms simply imports the gun, but Browning doesn't offer the pistol in the SFS configuration.
In a nutshell, here's how it works. A round is chambered and the pistol is cocked. The hammer is pushed forward with a thumb and the ambidextrous thumb safety engages. When the safety is disengaged, the hammer automatically springs into the cocked position for a single-action press of the trigger. There is no longer and heavier double-action shot to contend with. Trigger pull is consistent from first shot to last.
The most obvious way to recognize a Hi Power having the SFS system from the conventional is that the SFS Hi Power will have a very abbreviated hammer spur. The slide stop lever and thumb safeties will appear a bit differently shaped as well.
Downsides to the system is that the slide cannot be locked back for disassembly as it is with the standard Hi Power and it does add a greater number of parts to the pistol. More parts can mean greater potential for malfunction or breakage. Users are not reporting problems.
Do I intend to get one? Probably not but having said that I do not rather boorishly reply, "It's a solution to a non-existent problem" as have some others. I have no safety issues about Condition One Carry. Others do but would like to carry a Hi Power. The SFS system allows them to do that without the hammer actually being cocked. If the safety inadvertently wipes "off" on a single-action Hi Power, a press of the trigger is all that's required to fire the pistol. If this occurs with the SFS, the hammer is instantly cocked and it possible that this would alert the carrier to the mishap. Either pistol in a holster covering the trigger guard is still safe. If nothing can touch the trigger, a properly working Hi Power will not fire.
Other people are prohibited from carrying single-action automatics by policy. The SFS allows them to carry the Hi Power (or 1911) as it no longer to be a cocked-and-locked single-action. The hammer forward looks "safer" to administrators, city managers, and others not really competent to judge what is and is not safe.
For myself, the jury is still out on the SFS. I've not used one much at all (I've shot three) and I'd like to see how they function over time. Relatively few folks will use these guns compared to all those using Glock, SIG-Sauer, or HK handguns, so getting much long-term information/observations will probably take longer than with more popular handguns. I do think it may prove a viable and dependable system.
PS: Since this was published, an SFS owner/user contacted me wanting to make the article more accurate. Here is what he had to say:
"I read your article on SFS configured High Powers on your newer blog. You claimed no expertise on the subject, but did a good job of describing what it does -- except for one point. You stated that, with the SFS system installed, it was impossible to lock the slide back in order to remove the slide stop. This is a true statement, but it needed to be developed. One does not HAVE to lock the slide back in order to remove the slide stop. One simply cocks the hammer, push/pulls the slide stop out of the frame, and eases the slide off forward. I would add that it took about a dozen field strippings to free things up. My SFS-configured High Power is the easiest autoloader to field strip that I have ever dealt with. And at my age, this is a blessing.
For what it is worth, the Extended Slide Stop I had purchased from Brownell's a few years prior to getting C&S's SFS kit does exactly the same thing. In fact, it looks identical to the one in the SFS kit. Brownell's cost me $26 then, and slipped right in. It eliminated the need to lock the slide all the way back in order get the slide release out. I have no idea how it changed things, but it sure did.