Install
Follow the instructions.
The change of TTL from 64 to 100 indicates the router is listening for tftp connections, in the so called boot_wait state. You have to upload the firmware during the period when ttl=100.
Refer to the syntax on this page if you're running Windows.
If you'd rather use Buffalo's web interface to do the uploading, see here. You need to prepend the header.
Uninstall
I never tried this, but flashing back the original firmware(v2.32) should work as long as you delete the first 35-byte header.
Customize
Most network settings could be accessed through the web interface. You may also use nvram to get/set/commit the configuration values and run /sbin/wifi to activate your change.
See here on how to tweak the base system.
Packages
OpenWRT uses ipkg, a apt/dpkg-like package manager. Here lists the available repositories.
Howtos
And don't forget the numerous howtos, and mini-howtos.
Todos
Set up VPN.
Play with iptables.
Tips
There are a few other alternatives to OpenWRT, such as dd-wrt, freewrt, and hyperwrt.
Install miniupnpd (from backports) to better support MSN messenger. Unfortunately, miniupnpd_1.0-RC3 doesn't work out of the box. It seems to be using the obsolete "/lib/network" mechanism. I changed the line EXTIF=$(uci get network.wan.ifname) in /usr/bin/iptables_init.sh to EXTIF=$(nvram get wan_ifname). I also had to put ifname=$(nvram get wan_ifname) ipaddr=$(nvram get lan_ipaddr) in /etc/init.d/miniupnpd. Hopefully it'll fix the problem.
Use shfs to extend the router with external storage. Use dropbearkey to generate public/private key pair, and use "-i" to specify a key file. The command would be something like:
shfsmount --cmd="ssh -i /root/.ssh/id %u@%h /bin/bash" wh5a@adder.cs.virginia.edu:/home/wh5a/openwrt /opt
Publish the router's IP if the ISP dynamically assigns IP.
A few caveats: you may need to change the way current_ip is calculated. also make sure the web server can write to current_ip.
Now that we have extra storage, we could install more fun packages, such as apache, bash, and the toolchain(optware-devel), using optware here and here.
After all the hard work, of course you want to back up. For technical details, see here and here.