I forgot to mention earlier that the final Core rulebook (the DMG) came out on December 9. 2014, so the full set of Core books is now over a year old. And, according to all reports, the number of players (at least based on measurements from online gaming platforms that have players for a variety of RPGs) continues to increase. Amazon and Barnes & Noble sales numbers also remain high, even on the Core books.
As of this moment (January 8, 2016) on Amazon
PHB:
Average Customer Review: 4.5 out of 5 stars (1,181 customer reviews)
Amazon Best Sellers Rank: #135 in Books (See Top 100 in Books)
#1 in Books > Science Fiction & Fantasy > Gaming > Dungeons & Dragons
#30 in Books > Teens
[For comparison, Barnes & Noble lists it as Sales rank:520]
MM
Average Customer Review: 4.7 out of 5 stars (461 customer reviews)
Amazon Best Sellers Rank: #412 in Books (See Top 100 in Books)
#3 in Books > Science Fiction & Fantasy > Gaming > Dungeons & Dragons
#80 in Books > Teens
[For comparison, Barnes & Noble lists it as Sales rank:7,045]
DMG
Average Customer Review: 4.7 out of 5 stars (460 customer reviews)
Amazon Best Sellers Rank: #307 in Books (See Top 100 in Books)
#2 in Books > Science Fiction & Fantasy > Gaming > Dungeons & Dragons
#57 in Books > Teens
[For comparison, Barnes & Noble, lists it as Sales rank:1,349]
It's also ranked #1 on both Roll20 and Fantasy Grounds digital tabletop platforms (in both quantity of games being played and quantity of players), and #1 in game store sales by ICv2.