As mentioned in the Nethead roadmap, I have been investigating RIOT. It now provides a solid border router and 802.15.4 wireless networking. The GNRC networking stack is well done. The project also is very well run, and welcoming to newcomers.
I plan to advance with porting Nethead to work with RIOT, on SAMR21 Xplained Pro motes. I already have implemented a minimal CoAP client for the GNRC networking stack.
As part of developing the client, I ran across a bug with the layer 4 checksum. It was a pleasure to work with Martine Lenders to integrate the fix.