I once got a Decool Rotor from my visit from Hong Kong. I wouldn't say that the quality is on-par with Lego's but it's definitely better then other knockoffs. There were a few noticeable issues though.
Length 2 plus pins don't have the small "cuts" in between them, making it harder to remove them, and they were a bit "rounder" than a clean, straight, plus shape, giving it more friction and sometimes snapped my Lego technic connectors because of their larger shape.
Ball sockets have more friction then regular Lego sockets, but snap easily if used too much.
Some pieces were made of regular "soft plastic". For example the regular Inika shoulder armor from Lego had more of a "metallic" texture. When you drop it on the ground it would go "clink clank". But with the knockoff, it didn't make that noise, and were softer; I could make a dent in it if I tried hard enough with my nail.
Technic bushing were a slightly different shade of med-stone grey, almost pink-ish. They were thicker though, meaning they had more friction and were less prone to breaking.
Other technic connectors were far more easy to break unfortunately, and some of them weren't able to connect to regular Lego pins due to a slightly wider plus hole.
Despite these issues, I think that Decool is still the closest you can get to the real thing. 8/10