For $300, you can do a lot better. Even if you hate the Chinese handhelds, at that rate you're probably better off just getting a Nexus 7 and bluetooth controller.
While the Shield looks good, it doesn't seem worth the asking price, especially if you plan on using it primarily for emulation (which honestly does not take that much power). That's more than a 3DS, Vita, or even a Wii U.