I am doing some consulting for a small company and will be replacing their current router with a wrt54gl running x-wrt/openwrt. Main reason is number of firewall rules you can set. You see, they have in their dmz their mail and web server and a machine their clients log in to learn how to use the software developed by this company. The number of ports they need open and pointed to specific internal ips is more than the factory OS in the routers you can get at your best buy/newegg can handle. The fact they can edit their firewalls using a web interface just like using, say, the factory linksys one is very attractive to them as they would not need to have me there all the time. The fact I can go there and edit it through ssh as needed is convenient to me.

Second feature, which is more openwrt related, is they can separate the wireless from the lan. And then apply all kinds of ways to log into the wireless (and the lan, say, using a vpn). This way, clients can go there with their laptops and access the internet without compromising the lan. As soon as I learn how to deploy QoS rules that apply differently for the lan and wireless, I will be golden.
FYI, right now I am trying to find places I cal write about my experiences with x-wrt/openwrt.
