Trijicon recommends practicing to shoot on your first instinct as soon as your brain merges the dot that one eye sees with the image of the target area as the other eye sees it.
The more you concentrate the more difficult it seems to become. I seriously believe that's part of the reason why Trijicon settled on the whole "reflex" name as it is with said line of sights. Once you practice with the sight it's best to concentrate on the target and let your brain do what it is trying to do naturally.
The same effect is going on to a certain degree with other optics such as the Aimpoints and EOTechs but it isn't as taxing on the shooter as it is with a blacked out OEG type effect. The OEG is still useable and with practice can be both fast and accurate but it can definitely cross a person up if you put too much effort into it. If you have a red dot sight try it at the range the next time you are there shooting, might be surprised how many times you can manage to hit the same target even with the lense cap closed.
I bought my Trijicon NSN back around 1998 and have been using it for quite a long time, for out to 200 yards the thing works pretty darn well but the thing can be kinda finiky in certain lighting conditions and that's where the Reflex can act as OEG in some situations because to get a better dot contrast to target image you may have to close down the polarizing filter. There are quite a few threads on this in the optics forum but they may be buried now and in need of using the search feature.
I wound up buying an Aimpoint Comp M for a secondary sight about 2-3 years ago. In some situations I still prefer the Trijicon Reflex while in other situations I like the Aimpoint Comp M.
One downside to the OEG is that I believe there is a bit of parallax error when using an OEG type sight, atleast when I did tests with my Trijicon Reflex by shooting with 3 different styles, using the sight as you would a scope where it was full open and using the primary eye to sight/line the dot on target, using the sight full open while shooting with both eyes open concentrating on the target, and finally shooting with the polarizing filter closed down and shooting in an OEG mode. I noticed a bit of a shift in point of impact from point of aim between the 3 shooting methods.