the drawback to panniers is that they're not modular in their usage profile.

with a small high-quality rucksack/backpack, one can do other things with it:

A) cabin carry-on when flying
B) good for transport on a crowded subway without removal from the back (theft prevention)
C) good for a day hike
D) great for traveling in SE Asia where one gets a small locking box in each room
E) great for fitting into the smallest (and therefore cheapest) lockers at central train stations
F) going rock climbing in the hall or outside

i've found that 20L can hold clothing/lunch for two and a couple laptops or camera equipment, especially when it's designed correctly

roll-tops are really nice because one can put long items in such as baguette/steel bars and still shut the top.

just my .02€