Previews sometimes suck. Usually, even. A few months ago, just after Aliens: Colonial Marines came out (after having gotten pretty decent previews a few months before that, thanks to a deceptive demo), I asked my boss if we could stop doing previews. We all had a long discussion, and we decided against it, because it'd do a disservice to our readers to not provide them with the information that we get from those conferences. The solution, at least for us, is to be as honest and as candid as possible about what we're seeing and hearing, and to ignore the demos that suck or feel like a waste of time.
This asks the question, what is the point of a preview? If all you're doing is telling us what you saw, why can't we just watch an actual video of what you saw? What is the point of having a person there?
The answer is that the publishers/developers are using your credibility to make their advertisement seem like it's something else.
Now, I understand a website needs to make money, and not doing previews would essentially just be throwing money away as readers to to other sites to get them. However, making it sound like it's some kind of service to your readers you are proud to provide makes you sound either disingenuous or that you've stuck your head in the sand and pretending there is nothing wrong with reporting on advertisements.