I have GG&G, Troy, and MagPul plastic. The GG&G's are the nicest, and they better be, at about $280 a set. They are rock soild when deployed and when shooting for groups, and seem to offer the best accuracy. But not by a whole lot.
The Troys came with a S&W M&P 15 I picked up. They work fine, but are not in the same class as the GG&G's. They have a good bit of play in them when deployed, but the gun still shoots where I point it with them.
The MagPuls work fine as well, and are comparable to the Troys as far as accuracy goes. I have them on two AR's, and they have yet to have any issues.
If youre mounting a red dot, and its going to be your primary, I wouldnt waste the money on the higher priced sights. Other than to occasionally verify zero (the red dots do that anyway) I rarely ever use mine anymore. They are as advertised, "BUIS". In that case, the MagPuls fit the bill nicely, and will let you put more money towards a better red dot.
Im very suspicious of anything that comes at a cheap price, no matter how cool they look. There are a ton of knock offs floating around, most of which is probably "airsoft", and they are no way "quality", or anything close. I just recently saw some MagPul branded/packaged sights from China that were obviously knock offs. Pay attention, and buyer beware.
You really do get what you pay for. The reality of it is, only a rich man, can afford to buy cheap stuff.