Just wanted to share my experience with the Linksys WRT1900AC and hoping that it'll be useful to someone.
I purchased the WRT1900AC to replace the Fritz 7390 and needed something that would just work out of the box with NZ UFB settings (VLAN Tagging).
I was able to set it up in less than 5 mins and was up and running with my provider without any fuss using the stock firmware. The wireless range and speeds are truly spectacular. There are plenty of online reviews which cover the performance & features of this router so I will skip that here.
One major issue with the stock firmware is that when you specify custom dns providers, the router passes it's address back to clients as the primary and only dns. This of course was causing me a lot of issues at home as I couldn't use my Roku3 for certain channels.
There is a lot of information online regarding this issue and the verdict is that this behaviour is by design for the smartWiFi feature.
A few weeks back I decided to flash OpenWrt (McWRT by Chadster766) available on gitHub.
I have to say the router really comes to life with this firmware. I haven't had any issues with wifi or performance and the router seems to be very stable.
VLan tagging is really simple and the custom dns works as expected meaning we are back to a happy household!
If you need any specific info just ask