I have the Asus RT-AC68U/TM-AC1900. I bought it from T-Mobile for $60 and flashed it to stock and then put Asuswrt-Merlin on it (but you can use Tomato or DD-WRT). It does take a little work to flash it to stock (like Telnet'ing into the router), but it wasn't bad and for $60 I ended up with a wonderful router. There are guides online for flashing it back to stock.

If you're looking for information, I suggest SmallNetBuilder. They have very thorough reviews: http://www.smallnetbuilder.com/tools/rankers/router/view. It looks like the RT-AC68U is their #1 pick for AC1900 router now. It used to be their #2 pick under their previous testing methodology (after the R7000 Nighthawk from Netgear). That's slipped to #3 under the new testing and the Asus has taken the top slot.

Asuswrt-Merlin isn't such a radical departure from stock, but it has some nice features and allows me to do things like edit the etc/hosts to block certain things.

The Asus RT-AC68U is probably one of the top 2 AC1900 routers out there and T-Mobile is selling it for a song (even if you're not a T-Mobile customer). It's a little work to re-flash it so read a guide and see if you're comfortable with that. Or you could buy a stock RT-AC68U and get SmallNetBuilders #1 AC1900 router overall, for 2.4GHz avg throughput, 2.4GHz max throughput, 2.4GHz range, 5GHz avg throughput, and 5GHz range.