A bit of a bump but I finally bought this adapter and two Pro Controllers, one for me and one for the wife. I never used a Pro Controller before so here's a quick review of both items:
The adapter:
It was incredibly easy to use. I just plugged it in, pushed the sync button on the device and pushed the sync button on the controller. Done. I didn't download any drivers or anything. It just worked. I haven't tried the direct input mode yet, but the xinput works flawlessly. The first game I fired up was Super Meat Boy and I couldn't notice any lag. I got a couple A+s just to check it out and it worked great.
The Wii U Pro Controller:
Wow, where has this controller been all my life? My formerly favorite controller, at least comfort wise, was the Xbox 360's. The Pro edges out the win over the 360 for me. The buttons feel great and reset with a springy click. The thumbsticks also have that nice, quick, springy feel to them. Everything feels tight and well put together, similar to the 360 in that respect, actually. But it's this d-pad. This d-pad is incredible. It might be my favorite, but I've only used it for 30 minutes so I'll need some more time before I make that decision. Just to note, the Pro controller buttons don't rattle like the GamePad's which is thoroughly welcome.
I used to own a 360 controller until I gave it to a friend. That was before I got into PC gaming with a controller. I had been debating for awhile whether to buy another but they aren't cheap. I knew I would never buy a 360 so they would only get use with my PC. The real rub was that d-pad though. I never liked it especially since I'm partial to twitchy 2d side-scrollers. But, the 360 controller is considered something of a standard on PC now, so what else could I get? This solved my problem and I heartily endorse it.
In regards to how Mayflash mapped the A and B buttons to, well, the A and B buttons while the 360 has them in different places: since the thumbstick is placed above the face buttons on the Pro, this was a good move on Mayflash's part. You're still moving your thumb the same distance to hit the A button on both controllers. And, if it really bugs you, that's what the direct input mode is for. Customize them however you want.