also a happy rt-ac66u user here. i got mine about one year ago now.
within the first month i was on the merlin-wrt fw
then within a few more i had tried dd-wrt. at the time it was under developed. came back to it and tomato shibby around v116 in february and i have found tomato shibby to be my favourite FW for this router. currently running v120
i highly recommend the R7000 Nighthawk though because of its CPU. currently it is the most powerful router supporting tomato
it has even been overclocked from 1GHz to 1.4GHz and apparently its stable.
said by Toymaster:I was thinking the "R' meant like retail..so I was half way right, but still it is just confusing.
have you hand any issues with the 66U and what 802.11 do your devices connect to it with N/AC/B/G?
yes, the router is backwards compatible with all previous wifi standards a/b/g/n and obviously it works with ac
not to sure if thats exactly what you were asking, but for me it works well with everything i have tried. my only complaint is 2.4GHz band could have a bit better range