I have both the Eotech 512 and the Bushnell (that's the same as the 511). They both perform identically, no difference. However, the Bushnell is not a knock-off, they are made by Eotech and about $100-$150 less expensive. Supposedly, they are a non-mil-spec version, but I can find no difference in looks or performance. The Bushnell doesn't have the aluminum hood cover, maybe that is a difference......I'm not sure if the 511 comes with the aluminum cover.
There are some cheap Chinese knock-offs out there that aren't really worth the time, IMO. I'm a cheapskate and can't bring myself to buy one for that and other reasons.
I had an EOTech 512 and was pretty happy with it... until I got an Aimpoint Comp M2. EOTech battery life sucks.
I've messed with both the 511 and the the chinese copies. The chinese aren't a holosight. The reticle will move when you move your head. Also, I think the 1moa dot in the center is more like 3 or 4. Oh yeah, and the night vision button will turn the reticle green...
The chinese optic knockoffs aren't that bad. They are built pretty tough in my opinion. I wouldn't put them on anything that I would use to save my life though, and an M1A will destroy one pretty quick.