The ergonomics of this also depends on the laptop itself. The large Macs are sort of uncommonly good at being used on the lap as a result of lacking the numpad

The ergonomics of this also depends on the laptop itself. The large Macs are sort of uncommonly good at being used on the lap as a result of lacking the numpad and centering the keyboard/trackpad. My experience with trying to use my 15 inch Dell w/numpad on my lap is a bit more miserable, because I either have to position it off-center and hanging off my right leg, or type with my hands off-center, which becomes really uncomfortable quickly. I am tiny, though, so perhaps this would be less annoying to someone with bigger legs where it wouldn't be teetering off constantly.