I have two about three months old, one of which has been used fairly regularly, the other wasn't used until regularly until a couple of weeks ago. I am now using both daily.
I have seen no significant problems on either. Both of them have slightly too-high resistance readings, but that's been the case since they were new. And TC accuracy is still OK; not the best available now, but usable.
I have some doubts about the longevity of the 510 connector - one of them I already had to take apart, when it was new, to adjust the pin. But they haven't degraded further over time that I can see.
Also both of mine are batch 1, so such issues might have improved.
One issue that has cropped up: one of mine has a slight gap around the top plate, and I can't remember how long it's been there. The top plate is slightly pulling away from the body. I can almost certainly fix that very easily by removing/replacing the top plate, just I haven't bothered yet.
At the time I got mine, they were one of the best TC deals available - if not
the best. A 50W TC mod for about $40. However that is no longer the case.
There are now TC mods available for $35 with Titanium support and other advanced features - such as the Koopor Mini for $35. The IPV D2 is another consideration at $45. Both support Titanium, the Koopor also has FW upgradeability and a modifiable TCR scale, useful if you want to vape other wires besides Titanium and Ni200.
As such, while they are stable and have lasted, unless you can get one for $25 I would no longer recommend them over newer, more capable mods, which may even be cheaper. I see they go for $37 on FastTech, more than the Koopor. The Eleaf TC is only $30, albeit without native Titanium support (still usable with Titanium though, with temperature offsets.)
So they're now hard to recommend as a new purchase, unless you desperately want the low-luminosity, narrow-beam flashlight