I took a couple of classes and worked at the office of a pretty big time professor while I was in undergrad/grad school. Working in his office, I got to look at some of his LOR's and found them to be pretty plain. Since I did have the honor of being an office-female dog, I think I'd get an extra line about that. Will his fame offset the vanilla-ness of the letter? I'll already be borderline at some schools and was hoping this was one soft factor that could help push me over.
I think this would probably depend on how well you know the professor.
If he really likes you and seems eager to write a letter, then it might be worth pursuing. But, I personally suspect his fame won't offset a plain letter, and it might not be that helpful to send in a relatively plain letter as a way of strengthening your application -- by doing so, you implicitly suggest that you couldn't find someone else who was willing to write you a superior letter.
From everything I've heard and read, it's better to get a glowing recommendation from a TA who knows you extremely well than from a professor who is famous but doesn't praise you as an individual.
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