Deuter Race EXP. 18L and will take laptop shirt and tools. Won’t carry shoes or a full change of kit easily. I’ve used mine over the spring/summer when I’ve brought in a road bike, and it is a fine solution. I also use it off road with a bladder.
Now the weather draws in, it’s an Ortlieb Downtown pannier on a Topeak DX rack. The combined weight is an extra six kilos on my fixed road bike, which is 2/3 of the weight of the bike!
So hot and sweaty back vs. lugging a tank of a bike. It’s really not just “always panniers” simple, particularly if, like me, you shower and change at work.
Personally, I like riding my Carbon Defy with the rucksack, because if I’m off racing after work, I can leave most of the stuff behind. That bike weighs about the same as the full pannier and rack alone 😉