Variety's video game reporter, Ben Fritz, explained to Joystiq:
"I talked to one of our film critics and he said we usually do review the earliest available print, even if it's not the final version. If they're showing it at a festival to the public, for instance, we often review it. Even with the understanding that it may be re-cut or altered if/when it is released commercially. When that commercial release occurs we usually don't write a new review, unless it has been hugely changed. Sometimes we actually talk about the fact that it may be changed before commercial release. Check out our review of "Che" from the Cannes film festival, where the first sentence of the review starts "No doubt it will be back to the drawing board..."
The basic rule is: If they are showing it to the public in any way shape or form, we feel justified in reviewing it. The only time we wouldn't is if they are explicitly showing a "work in progress," like a test screening or a series of clips shown to exhibitors or something like that. That would be like running a review based on an E3 demo. Another relevant analogy I thought of is that we do review Broadway-bound shows when they are doing a "tryout" in another city. However, in that case, we usually review it again once it opens on Broadway.
So, the short answer is, if Fable II were a movie or play, we would absolutely review it the day it opened, even in its "incomplete" form. Which is why I have decided I will do the same with this game. Luckily in the age of blogs, however, I can update my thoughts when online co-op becomes available."