I love Martingale collars and use them exclusively as our "everyday" collar. When we do shows or obedience trials, then we use the appropriate collar for those events (and the dog knows in advance what we will be doing because he or she can feel the difference!)
I hope I can give some confidence to the reviewer who felt that a Martingale collar is not safe to be left on as a “regular” collar. The Martingale was made as a safety collar with just that situation in mind.
In many collars, those with a regular buckle, or clip fastener, and especially a plain choke collar, a dog can unfortunately get caught on something and can choke as his collar get twisted and tighter. It's a heartbreak.
Martingale collars were made to function as an ordinary choke collar in that, while walking, you can “pop and release” the collar to get the dog's attention focussed on you. (“Pop and Release” is the proper way to use any choke collar: you pull the leash very quickly so that the collar tightens, then you immediately loosen the collar.) A Martingale allows that quick tightening, but, because of the construction, will only tighten a certain amount and no more. This means that the collar can't get too tight and choke the dog if he gets it stuck on something. It also means that a dog owner who doesn't know how to use a choke collar cannot pull it too tightly and leave it tight as the dog pulls along, gasping for air.
The best way to learn to use a collar and know how your dog feels when you walk him or her is to practice on yourself. Thankfully, you don't have to use your neck; your arm or leg works fine. Feel the difference between a Choke-and-- Keep-Choking vs. a Pop-and-Release. Try on a Martingale, fit it properly so that it will tighten just enough around your chosen limb, and try to pull it hard enough so that you can pretend to choke your arm. It's impossible.
I'm sorry if I've gone on (and on...) with information that almost all of you already know, but if it will help one dog, I'm willing to be a windbag.