I liked the 20l one of these, till I fell off and it wasn’t waterproof any more.
So I got another. It had a waist strap, which I found a bit restrictive (you can clip it round the bag, rather than yourself, or just cut it off), and I didn’t use the side straps, cos they buzzed in the wind. Plenty of room for shirt, work papers/laptop, lunch, shoes at a push, etc. lasted through several Scottish winters and still going strong. Totally dry – like not even a hint of damp.
Back ventilation wasn’t its strongest point, but there were showers at work, so I put up with it – it didn’t really seem to be an issue on the bike to be honest.
Yellow seemed the obvious choice for visibility.