Depends on the airline and the configuration, of course. On UA, I flew in the upper deck because it was more private, less crowded, and much quieter than the lower deck business class cabin. I also like the storage bins by the windows. And it IS pretty cool going upstairs on a plane!

On LH, business class is the front quarter of the main deck. I sat in the nose section (row 2), which is somewhat separated from "main" business class section by a center lav and the exit rows. Again, felt much more private and was definitely quieter. Wasn't a big fan of the seats, but the front section itself was nice. Not as "cool" as going upstairs, though (where LH puts F).