OK, so I picked up a Buffalo WMR-433W from Yodobashi on the way home from the office. My expectations dropped a little when I noticed that it had a 2.4Ghz/5Ghz switch on the side of it, implying (correctly) that it can only do one band at a time, but for US$13 I still had to buy it...
First impression on opening it is that it's tiny, and light (they claim 19g, and I'd believe it).
In the box was the router and a very short USB/MicroUSB cable - no actual power adapter. As the box promised, it also included a quick-start guide in English, although it's very "quick" in that it only covers using it as a wired router and nothing more.
The quickstart guide is the ONLY thing that's in English. The slightly more complete manual that comes with it is in Japanese, as is the GUI, with absolutely no option to change that, which makes configuring it somewhere between difficult (for someone that knows how these devices normally work) and impossible (for anyone else).
With some trial and error, I was able to get it working in WISP mode (or what the GUI calls, according to Google Translate, "Wireless One" mode) on 2.4Ghz. I flipped the switch to 5Ghz, which changes the SSID ("Buffalo-A-xxxx" rather than "Buffalo-G-xxxx"), and not surprising it lost the WISP link, however I was able to go in and reconfigure it again (thankfully the Hilton Osaka does have 5Ghz!), at which point Speedtest gives me :
Remember, that's WISP mode, so it's doing both up and down over wifi!! Apparently in WISP mode you can use the Ethernet port as a LAN connection, but I didn't actually try.
The GUI is very clunky. Not only is it in Japanese, but everything is wizard-style based with limited options. For example there's no option to set the SSID when configuring it in WISP mode. Instead you need to configure it in router mode and set the SSID there, then reconfigure it into WISP mode at which point it will keep your old SSID. You can't change IP addresses (defaults to 192.168.13.1), DHCP scopes, or basically anything else.
I did manage to find a firmware upgrade (from 1.51 to 1.52), but even that was still Japanese only and nothing seems to have changed.
Next I configured it in "ルーターモード" (which according to google translate is "router mode", for those of us that don't read Japanese) using the hotels wired internet, and the performance was about the same as for WISP mode.
At this point the only really issue is that my Chromecast doesn't work with it - simply because it's 2.4Ghz only. I'd been looking for an excuse to buy a new Chromecast, so this is probably it. Everything else I travel with is 5Ghz. Of course, you can only use 5Ghz for the client side if the hotel has 5Ghz, or if you're using wired upstream. If you're trying to WISP a 2.4Ghz network, then you're stuck on 2.4Ghz for your network too.
So basically
Pros : Small, light, 5Ghz, fast, cheap (US$13!), seems stable, but it's really too early to tell.
Cons : 2.4Ghz OR 5Ghz only. Feature poor (no USB, OpenVPN, etc). WAN or LAN port (depending on mode), but not both. Difficult to use (due to language) but very simplistic GUI. Can't change config on most anything.
All up, given the size, I'm pretty sure this stay in my travel kit. Only time will tell if I prefer it over my GL-AR300M, on basically a function v's speed curve...