Just got back from a trip to Europe and picked up one of these at a bike shop. I've only used it twice, but figured I'd throw in my 2 cents.

Overall, I like it. My first day out included a spare tire, tire levers, pump, jeans, socks, boxers, belt, polo, hand towel, undershirt, flip flops/shower shoes, and my 13" laptop, along with a set of Campy Record brakes that I was selling on Craigslist! I probably had a bit of space to spare. It was heavy, but that was only a factor when I had to stand up on the saddle for a couple of hills. The affect on balance was noticeable. Besides that, getting on and off the bike felt a bit awkward with the extra weight, but that wasn't a big deal for me. The next time I used the bag I didn't have the laptop or the brakes and it felt more manageable off the saddle.

This was my only commuting experience (approx 12 miles each way, heat and humidity during summer months) without a backpack and I really prefer this method. I previously had been using a nice osprey pack with a steel frame to keep it from sitting directly on my back. I was far more comfortable and cool using the saddle pack, and felt noticeably less fatigued on my rides home without having 15 pounds on my back.

Downside: Only two at this point. The first is the balance issue when getting out of the saddle as discussed above. The 2nd is the fact that I ride small frames and have my seat raised considerably high, even with that height the bag eventually ends up sitting on my seatpost binder. I think that over time that will damage the paint. So, I'm trying to think something up that either covers the area where the bag touches, or keeps the bag off the binder bolt completely. If I ran a shorter seat post this problem would be exacerbated.

EDIT: 1 other issue - there aren't any smaller compartments inside or outside of the bag. So, smaller items have to be thrown into the large space vs. they're own zippered compartment.

https://www.apidura.com/product/saddle-pack-regular/