Valve, in an anti-MS spite-based move in not offering a Windows version created a significant hole in the market, which it looks like ASUS has addressed. Good. I hope this device does very, very well. Neither my kids nor I have much need for a portable gaming device right now (we have Xboxes in various rooms), but if we did, I would grab one or more of these based on what I've seen so far.
Having said that, I still hope MS releases a portable system that ties more directly to the Xbox ecosystem, running games in my Xbox library over my Windows gaming library. Even better, combine both! That would be so appealing, I might have to buy one even without a need for a portable system.