As a new AR shooter on a budget, I was looking for inexpensive metal BUIS for a M&P15 and bought these:

http://www.amazon.com/gp/product/B004SH1I7U/

http://www.amazon.com/gp/product/B001WIUJXW/

I've only shot with them twice, but they seem to be solid and hold zero. So far, anyway... Time will tell.