Everyone knows the guy has the talent to pull it off. But we aren't sure about his look. He looks too young. Fan favorite Viggo Mortensen looks a little older. Also, Bardem is a little too filled out. A little too lardy. He'd have to lose some lbs to play lanky, often starved Roland.
And lastly, Bardem has a strong Spanish accent. We don't know how that is going to work with the rest of the cast. It's possible that they will give everyone in Roland's original town/world a Spanish accent. In the fourth book, Roland spent a lot of time in a town where people spoke a little Spanish. That could work.
We didn't see him in the role at first, but we will give him a chance. We'll judge after we see the performance.
EDIT: The project has been dropped by the network and is now being shopped elsewhere.