I've been using Tomato on my RT-N16 for about half a year now, and I have to say it is the most stable router I have ever used (this is my first foray into 3rd party firmwares).
I've noticed that on the far side of my house, I can sometimes have trouble picking up a wifi signal though. Walls and distance being the obvious cause. I figured getting better antennas would help, but I'm having trouble finding any information on antennas that people have used with this router that resulted in a noticeable improvement.
I found this on Ebay: http://cgi.ebay.ca/7dBi-RP-SMA-Antennas-3-Asus-RT-N16-Gigabit-Rtr-/350451862972
They say these are specifically tested with the RT-N16, but I'm a a bit wary of ordering them blindly.