Love em. Easy to clean, easy to do up tight (or do up on a fat horse ), never had slipping or rubbing, doesn't seem to pinch skin. I wouldn't use anything else. I use the breathable sort and seem to get less girth sweat than usual.
Old trainer had one. They are pretty good, they definitely don't slip. The onl problem I saw was that they do sweat up pretty easy, and they make the coat a little coarse where it sits, even when they are dry.
I looked at my horses girth area today, he's clipped, and thought I could see a rub on both sides.....first time it's happened in five months with a neoprene, we worked hard, but no harder than usual.....will give him the day off tomorrow and see how it is on Thursday.....
Some horses tolerate it, others not so much. Been using one for 4 years, love how easy it is to clean and take care of (got enough to do, if I can cut back my work load I will). That said some horses react badly to it, that's usually when I switch to my handy fleece girth kept hanging in the tack room.
I do not like the waffle pattern neoprene, had one and it DID rub my main trail horse raw, but the "solid" ones work just fine and have never bothered her. Waffle pads also rub her though, so I think she is one of the horses who doesn't react well to waffle patterned tack. I have a weaver neoprene cinch right now, and so far it is standing up pretty well, hasn't sored anyone up and has a nice "clench" to it while still being flexiable so my mare has no constriction to her breathing.
If your horse has a minor rub from a cinch, you can put vaseline on a neoprene cinch and keep riding him supposedly. I wouldn't do it, but I have heard you can.
I use only harness leather western cinches on my saddles. They work great for me. I keep them very clean and oiled well.