I've installed a couple Wiikey's. Takes longer to open the case than it does to install the chip. So far everything works just as it's supposed to. Only GC homebrew currently runs. I imagine Wii homebrew to be quite a ways down the road.
The new Wii motherboard doesn't affect anything. You'll just have to run some wires - which is stupidly simple if you've ever used a soldering iron before. I've installed the current Wiikey chip with wires.