It's pretty safe to say that most people know better than to trust professional reviewers of just about anything these days. That said, though, it still might be somewhat useful to understand why that mistrust in them abounds.
"I like what I like." It's a simple enough statement, but it covers the entirety of why a professional reviewer's opinion fails compared to that of a friend. With a friend, you have a general understanding of their likes and dislikes, whether or not you happen to share them. Say, for instance, that a friend of yours absolutely hates horror movies, and you've loved to death every horror movie that he's hated. If somehow, he gets dragged along by another friend to see the latest, and tells you that it absolutely scared him out of his wits, that's probably a good indication you'd better get out to see that show.
The professional, meanwhile, is tied up in trying to be a professional. There need to be reasons that a professional does things, whereas an amateur can usually get by doing them by feel. (A professional, after all, has to justify why he should be paid, particularly when it comes to something like watching movies for a living.) So, our pro here has to keep their eyes out for the direction, and action, and the various story elements, and try to make a rational decision and commentary based on those things.
Where's the disconnect? After all, a well-directed, well-acted, well-storied movie ought to be a hit, right? Well, here's the problem in a nutshell. People, by and large, aren't going to the movies (or watch television, for that matter) for the art. They're doing it to be entertained... which, naturally, is an entirely different level from watching something for artistic value. Take Independence Day, for example. The professionals couldn't pan the thing fast enough. A pulp movie if there ever was one, aliens and rayguns and the potential destruction of earth averted by a cable TV guy. And yet, the viewing public couldn't get enough of it. Not because it was a great movie, but because it was an enjoyable movie.
It runs the other way, too, with all the press and pre-review for things. Take, for instance, all the pre-canned buzz that Studio 60, or whatever it's called, got from NBC before the season started. And yet, what took off in the eyes of viewers wasn't that, but the relatively unreviewed and unhyped (pre-season, remember) Heroes (which, unsurprisingly, is one of the two or three TV shows in English that I'll actually watch).
So, who are you going to trust when you're trying to make your media consumption choices? Seems like a fairly obvious choice.