I've been trying all the ambi thumb safeties I can, trying to find the best one for left handed use (and I have some very exacting, perhaps OCD standards). I've tried several now, and here are my experiences with each:
Mueschke: I've had two of these. They are made by (I believe) the MIM process. They are retained by the Colt method, an extended sear pin. The joints in both examples were sloppy and eventually broke. The sear pin is supposed to be weak as well, and the reason Colt abandoned this method of retention. I'd give this one star out of five.
Kimber: This is a quality thumb safety made via the MIM process. It is retained via a hammer pin. The joint is sloppy and needs to be tightened. The safety works as advertised. However, mine came from the factory a bit rough, and after I stoned it smooth, I noticed the plunger begin to dig into the safety, bringing everything to literal grinding halt. I did not know at the time that MIM can only be case hardened, and I had gone past the hardened part. A bonus is that it will usually drop into guns which are in spec. I'd give it three out of five stars.
Auto Ordnance: Believe it or not, this thing was probably my favorite. It appeared cast. The joint had absolutely no play as the safety was retained on the frame by a #4-40 screw that goes through the shaft. Very clean install. However, this was also its undoing: The design is supposed to be weak (I didn't experience this) and will only fit thinner frames. As well, it was not well finished. It was too tight on my carry gun, and so it went on race gun I had built from spare parts and subsequently traded to a gun shop for a 9mm carbine. But at around $16.00 from Numrich (Welcome to Numrich Gun Parts Corp.) it may be worth a shot if you're looking for something cheap to try. Two out of five stars, due to fit on some guns, and its poor finish.
King's Ambi: According to the factory, this safety is cast and looked it. Retention is via a hammer pin (King's method). The joint is sloppy, but can be tightened. I did run into a few problems though. First, the safety allowed overtravel of the sear. Without going into the workings of the 1911 too awful much, this allowed light strikes on the primer due to things rubbing the hammer and slowing it down. This would be an excellent safety if my gun were equipped with a trigger that has an overtravel stop. I will probably reinstall this one on my 1911 after I acquire an EGW trigger (fitted, but no screw to back out).
STI/SVI: This is retained by the Swenson method; there is a tab which rides under the grip panel. This safety is machined out of stock, and I've had mine the longest of any of these other safeties, on two different pistols. I can't break it. The joint actually snaps together with an audible click, and though it will ride out a bit, this can be fixed by tightening the groove even further. I ran it for a bit with no support on the right side, then carried it with Pachmayr grips (mushy), and the shaft torqued and sprang back with no complaint - and I really ride safeties hard when shooting. The stud has plenty of material to work with, unlike any other safety I've fitted. In fact, to get it through the frame, I had to take material off the circumference of the stud. When disengaged, the stud serves as a positive stop for the sear/trigger, making it feel almost like a fitted or adjustable trigger. From the safeties I've tried, this one gets four out of five stars: I dislike the Swenson retention style.
I have not yet tried an Ed Brown, Wilson Combat, or any other.
If any other lefties wish to do some write ups and add to this thread, please feel free. I'm posting this in the hopes that my time and money will help others who are do not have my penchant for experimentation get a good product which fits them the first time, or at the very least, the second time.
Additions, as I said, are welcome. I'll add more myself if I do any more "playing around."