Have STT Pros on the wifes GX. I am going to say good overall tire and never had a minutes trouble from them. That said, if you are doing mostly street driving and if you plan to try and run them for a while, they do not wear well (well ours haven't). I don't know exactly how many miles we have on them, but probably less than 50k, but they are by far the loudest worn tires I have had, and I took several sets of the original STTs and some OG BFGs to 70k easily and even cupped badly, they were not this rough or loud. Any MT will get louder as they age, but these seem to be worse than others.
From a strength and reliability standpoint, no issues and I would recommend them, but with all the new offerings on the market, and since most folks I would say are not prepared for the way these tires wear, I could see where folks who were all in on a MT decide quickly that an RT or AT would better suit their needs. Also, think flavor of the month, kind of like the Duratracs. For a while that was all anybody talked about, then something newer and cooler came along and they had to get rid of them because of (fill in the blank) and get the new wizbang tire. I think it is cyclical as much as any other fad, and when you are talking about "needing" to replace a relatively new anything, you will find a way to justify it to yourself, like, "oh the sidewall is weak, I don't know why it couldn't handle that razor sharp rock directly in the side on a downhill cross-slope" I just need to get some new wizbangs, they would have been fine. (100% my opinion on that part)
The GX will likely be getting a set of Ridge Grapplers moving forward, but that is because of how it is used, not because of the tire itself. If you are happy with them, then I'd say get another set, that is how I ran out 2 sets of original STTs, and currently am still running on a set on one of the trucks.
As far as the weak sidewalls, etc. I hear that from lots of tires, but many times it is more about the situation, and has less to do with the tire. If you are looking for a strong sidewall, I am going to say the Toyo MT is probably one of the toughest out there, but they are spendy, heavy as hell and do not have much sidewall flex. As far as other options, if you are looking for a good all around MT, I have seen/heard good things about the Falken MTs. We currently have Mickey T's, Toyo MT's, STT Pros, OG STTs and a set of BFG Reds on various trucks and as far as a MT, for streetable rig, out of those, I would still probably pick the STT Pro for a general use rig, but I will say, I am leaning toward the newer RT tires as a good stop gap if you want something a little more aggressive, but to still have good road manners.
Bottom line, I think Cooper makes good tires and I would buy them again. The thing you have to remember with all of it is that you will always hear/see the bad reviews, it is not as often that you see the "I'm a pleased customer who will go this route again" comments. Think about how many times most folks don't compliment the manager of a restaurant for the good service they got from a waiter/waitress, but those same folks are real quick to ask for a manager when something isn't to their liking. Just human nature, or so I'm told.
