EFI-X comes in the form of an USB dongle with firmware on it that allows you to install Mac OS X (and other operating systems) unmodified, unpatched, and unhacked, on supported regular computers. Since Mac OS X does not need to be hacked or patched in any way, updates from Apple work without any problems; Gizmodo installed Mac OS X 10.5.5 without issues.
There is a catch, of course. The boot process is slightly awkward, since you first need to boot to the USB dongle, and then continue to booting Mac OS X, giving you a boot time of about 2 minutes. Once you're there, though, Gizmodo claims, there are no problems. No compatibility issues, no weirdness, and full, unlimited performance.
EFI-X comes at a price, of course: USD 170. The company behind it claims, therefore, that EFI-X is not a cost-effective solution - it's for enthusiasts. "EFI-X is not for everyone. It is not for who wants to save money, at all. It is for enthusiasts that put expandability and extreme performances before anything else in their computing needs. We heard those voices, and we answered."
There is no response from Apple, yet, concerning EFI-X.