The S&W M&P15 has become very popular due to its low cost and availability. My personal experience with owning one was that my M&P15R had an incorrectly cut upper that would not hold the ejection port door closed, the stock was not installed correctly and was crooked, and the fire control group was defective and would double. S&W replaced the lower on my rifle, and I did not bother to have them work on the upper.
Often at the range I have seen people have minor issues with M&P15s. The new low cost model (around $700) came with a near useless rear sight. This cheap copy of the detachable carry handle often would not index correctly. Last Sunday, I saw this issue. This new, out of the box, M&P15 came with a broken bolt catch. While I have no doubt that S&W will fix this, I have been less then impressed by S&W quality control on their rifles.