I got this Buffalo WZR-600DHP router thinking that it was a worthy successor to the WRT64GL router, which radio stopped working on me over the weekend. The Buffalo router also had DD-WRT installed, so I thought it meant one less step to install. Turns out to have been a 2-hour pain to configure.
1. The default username/password didn't even work, trying every single combination of admin/password, root/password, buffalo/dd-wrt123, etc. that was possible. I had to hold down the Reset button for 30 seconds to reset everything back to a default state. (If you hold down shorter than this period, I often had issues resetting the firmware).
2. Soon after tweaking the IP address to something other than the default IP address of 192.168.3.1 and rebooting, the Buffalo router started to create 302 redirects on its web interface to 192.168.11.1. Sure you could use the router so long as you never needed to touch the admin interface. Was there something in the firmware that has some hard-coded setting? Or is it running a DD-WRT version that makes assumptions about its IP address?
3. The solution for both items was simply to replace Buffalo's stock DD-WRT firmware with the latest one available. Make sure to "reset to default settings" when uploading the new firmware to clear out the old stuff setup by Buffalo. Buffalo's stock firmware also prevents you from enabling SSH into the router, which makes even harder given problems with #2 to troubleshoot.
4. Don't close any windows until the firmware is flashed.
5. Connect to the router at 192.168.1.1. If you're using DynDNS and setting up with afraid.org, keep in mind that the hostname specified needs to be hostname,hash value (i.e. xxxx.mooo.com,abcde). There's still a bug in DD-WRT with the hash being truncated (http://www.dd-wrt.com/phpBB2/viewtopic.php?p=781615). You can go to http://freedns.afraid.org/dynamic/ to copy/paste the right hash value from the direct URL (i.e. http://freedns.afraid.org/dynamic/update.php?HASHVALUEHERE)