I commute 9 miles each way and have been using this bag for about a month. It's large enough for a complete change of clothes (I roll my clothes, rather than folding) OR a lunch in a container, plus phone, keys, wallet. Unfortunately it's not big enough for a change of clothes AND lunch. It's easy and quick for me to drive to work on the weekends so I usually drive in on the weekend and drop off clothes for the week. This allows me to just carry lunch and my personal items each day.
The bag fits very snug on the seatpost and saddle rails and has absolutely zero movement, even when standing and pedaling hard. I've also taken it on a five mile ride on rough gravel roads and it did fine. With it being winter, I have not yet commuted in the rain, but I've used Ortlieb products in the past and I have zero concern over its waterproof-ness.
I was debating between this bag and a larger bikepacking bag such as the Revelate Terrapin or Porcelain Rocket Mr. Fusion, but I was worried they would be overkill for my application. I kind of regret not going with a bikepacking bag but this will work just fine for my needs. Ideally I'd have something with a capacity a little larger than this, and slightly smaller than a bikepacking saddle bag.
On weeks when I cannot make time to drive into the office, I use a large backpack (Osprey Radial 34) and carry my clothes and lunch.
I have the Pika and the Outpost to compare this to, and the Ortlieb beats them both. Those are $120 and $99 respectively, however I really do think you get your money's worth if you spend a little extra and get this one. All straps, buckles, and the bag body are very high quality. The air valve is extremely well designed, I hope ortlieb starts to include these on all their waterproof touring bags. I've used it on one short two week tour right before the cold season hit and it worked beautifully. At first I used it solely for my clothing, but unless the clothes are packed down very tight the bag sometimes gets floppy. I'm a lazy packer, so I started to put my tentpoles in along with the clothes and that helped the bag keep its shape better. Ortlieb seat pack and two back rollers on front pannier racks should give you plenty of space for touring through countries like the US, Canada, Japan, etc.
Ordered one for me and a friend so we can haul some gear on our rides. We end up in places where it would difficult getting a ride home should anything happen so we go prepared with tools, extra clothes and a DSLR on occasion. Pack works very well and can hold more than it looks. Haven't gotten it wet yet but the material does seem very waterproof.
Wanted a ride wallet to carry phone, cash, CC, etc. This fits the bill but the execution could be better. The transparent pouch zipper is difficult to pass things through (to small for an iPhone) and hard to get things out, especially if wearing gloves. The zipper would have been more helpful lengthwise.