I am running Tomato Firmware 1.28.0000 MIPSR2-102 K26 USB AIO-64K on Asus RT-N66U. I had used Tomato firmware in the past on an older Asus router too. While it is a great firmware, it was always complicated to know how so many different settings respond to changes.
I had an Engenius router before but bought an Asus RT-N66U in order to be able to run Tomato on it. The main reason for running Tomato is to be able to watch download/upload per IP address. I am happy that Tomato does provide bandwidth monitoring per IP address but there are issues and problems:
1- The router does not promptly update the list of connected devices. For example, if a laptop or smart phone was on the network one month ago, it is still on the device list and I don't find any way to update the list to remove old devices from the list.
Even worse: when a device connects to the router and the router assigns an IP address to the device, it sticks to the device so that if I assign a different IP address to the same device, it does not switch to the new IP address right away. What happens is that the device now shows in the device list twice with two different IP addresses, one assigned by the router and one assigned by me through the static DHCP. Both IP addresses also show up the bandwidth monitor per IP.
2- It seems sometimes the router slows down and it takes me some time before a web page opens. I didn't have such a problem with the Engenius router or with the Asus with the stock firmware.
3- When a device on the network starts downloading a large file, the general web browsing sometimes slows down. The Engenius router didn't have this problem and the stock Asus firmware wasn't as bad either.