How much I enjoy a long common route naturally depends on the quality of the visual novel. As long as I don't find a visual novel boring, I much prefer a long common route to a shorter one. A long common route gives more time to world building, character building, and makes for a more organic segue into the character specific routes (or at least generally speaking). If I had to pick one over the other, I guess I would say I would prefer a game to have a long common route as opposed to a short one. Ultimately, you can always use the skip function or load a save after all.
I enjoyed Grisaia's common route in particular. The humor never fell flat and each scene developed my impression/interest of Mihama Academy's student body. This might sound strange, but I was actually a bit bummed when the common route concluded.