so... you have no opinions on the car you drive, right? because you couldn't design and make one yourself, you have no right to judge if say a kia is better than a BMW, right? Or if you go to a resturant and order a meal, you don't have any opinions on the taste or quality, since you're not a professional chef?
Once someone puts something out there to be consumed and asks MONEY for it, we as the CONSUMER have every right to critique and judge for ourselves, whether we like it or not. What gives us this right, because we're putting out hard earned money down.
And the reason people like Ebert get paid to offer their opinions? Because other people *gasp* like and even respect what they have to say. Maybe you don't, that's fine. Let's see you do better.