Decent Value, So-So Design
Technically speaking, there's nothing to complain about with the MBUS; they're easy to install, they zero simply enough, and the fast and flexible deployment options (just press the center or either of the side flanges) are a welcome feature. However, the idea of sights is to allow reasonably fast and accurate aiming in the field, and a couple of areas of the MBUS design don't lend themselves all that well to this purpose. Specific to the rear sight is the square profile of its "ring" - the holes are round, but the pieces into which they're drilled are square. While this may seem immaterial, in practice the shape clashes with the roundness of the hole, doesn't do as much to draw in the eye, and, thanks to the extra material at the corners, blocks more of the space around the target. For me, this makes aiming slower and clumsier at defensive ranges, at least when using the more accurate narrow aperture. The flip-up piece used to change aperture size isn't quite as fast as a traditional rotating head, though it does help minimize the sight's flipped-down profile for those who use large-lens optics. And this particular color (FDE) was found to reflect more light into the eye from the posts flanking the aperture, and this sometimes added to the element of distraction (although the contrast did help isolate the black center piece, so I guess it's a trade-off). So if you really just want an affordable but decent-quality backup unit, you could do a lot worse. But if your irons are more than just a last-ditch measure, I'll suggest springing for a nicer set.
Decent but not great
I'm a big fan of Magpul hardware, but this fold-down backup sight was a little underwhelming. It does not seem as sturdy as I would have liked. It is functional and works as advertised but I have reservations about the long-term usability of this item and whether or not it will stay in one piece and hold zero over the long term. It was very difficult to flip down the front peep aperture, so much that I almost broke it. I know Magpul's area of expertise is quality injection molded plastics, but sadly I have come to the conclusion that there is no substitute for quality all-metal sights.
Be Aware
This sight will NOT co-witness with a standard A2 front sight. If you have a Bushmaster Dissapator type rifle, look elsewere. I still haven't found a rear sight that will line up correctly with a permanent front sight. Good quality sight but needs an adapter to raise it up to the correct level.
