We are in the OEM-handles-well camp with our E450 chassis. We have a long 220" wheelbase, which I've seen mentioned as making a difference, so maybe that's why we don't have handling problems. It certainly doesn't handle like a race car, but I also don't feel like I'm ever fighting it like some people mention.
We also don't have cab comfort problems, but neither of us are particularly big or tall people, so that might be a difference there. I do love the seats and literally comment on the comfort almost every time we go somewhere.