I also find that most HGV drivers are pretty decent. When you've got that many gears, and slowing down/speeding up takes such a long time, theres no point getting impatient. I try to be nice to them, they're usually nice to me (one time I got a lift home and a cup of sweet, white tea from one of those guys, after a white delivery van had come out of a side road and t-boned me).
But like any kind of road user, there are some right wasserks too.