I've smoked every grade of brisket there is and results have been pretty close regardless of the grade of meat. Just the entire process of smoking it low and slow until it is so tender makes it not vastly different between grades. Of course there is more marbling with each rating increase which does add to the flavor but it really isn't a huge difference.

I was looking at them at Costco yesterday and a 15# (approximate) prime grade brisket was well over $100.00 so I'm going to pass on buying any until the price comes back down.