Items that I use daily ... like a backpack I really like it high-quality ... for me a pannier wouldn't work and a 20L back-pack is perfect.
up at 5am.
on the bike at 7am to work with change of clothes, lunch, Retina MBP, wallet, passport, phone, headphones and sometimes an extra item.
jam bike into crowded locker (for faculty only!) where kitty litter buckets would get snapped off for taking up too much space
into gym to lift and shower with bag into locker (random locker assignment every time, no long-term lockers)
into office and unload backpack ... set-up computer
run around campus (sometimes backpack is needed for computer or other books)
to climbing hall inside with backpack
backpack lies at base of wall with some stuff in it water bottles, etc...
into shower room and locker
back on bike
ride to market and buy food with backpack (roll top is good for baguettes and schnapps bottles poking out the top)
back to flat
some days couple that with a ride on a bus between campuses (50min bus ride on a crowded bus)
or riding bike to airport and departing for flight
for me ... panniers are a hassle and aren't comfortable to carry on my back, they're not comfortable when I have to have them on my lap for a bus ride and they don't always fit in an airline seat or go through security easily (although we don't have the TSA thankfully).
for me a high-end 20L backpack is perfect ... i can travel a week out of it fits enough clothes ... it fits a computer really well and it's super durable and doesn't attract unwanted attention.
€90 for a Spear 20 was more than reasonable in my opinion.
Ortliebs would be OK if I wouldn't want to fly with them ... but panniers would be more of a hassle.
if one just locks up their bike in their garage and at work then cheap panniers are probably more than fine (or even a kitty litter container).