As for it replacing the M4/M16 as fishbed said it's a long long way away from that. It's not just a question of better weapon system here but does the Gov't really want to spend all that time and money required to replace the existing weaponry for something that does the same thing.
Just to clarify, the Israeli military HAS adopted the TAR-21 as their standard infantry rifle (back in 2009). But adoption has been slow (the design has been around since 2001), and there are still large numbers of M4s still in Israeli service.
I'd love to have one, for sure. It seems to be one of the most well-thought-out bullpups on the market, and has over a decade of development behind it at this point.