I use a rucksack to commute, but (a) it's only 2 miles each way, and (
all it has in it is my lunch (1 sandwich, 1 banana and/or apple, 1 biscuit). This is because my road bike has no eyes for rack and panniers, and the wedge bag is only big enough for lock, spare inner and few tools. I did try with a bigger Carradice bag for a while but got fed up with the way it sagged.
Laptop in a rucksack: I have tried but I find it most uncomfortable: certainly not for long distances.
I should also warn anyone with back problems - or potential back problems - to be wary of frequently carrying heavy loads in a rucksack. I think that's how I gave myself the sciatica which has been plaguing me for years...
My son has used one of those topeak racks which fit only to the seatpost and doesn't need eyes in the frame: the sort that takes a bag that slides on; but he's had problems with those too. The first one he fitted snapped after a few weeks - despite the fact that his load was well within the recommended limit. He got a refund under warranty and replaced it with the next size up, but it was never really convenient to use.