They are nicely designed.
It depends on what you carry adn what other bags you have. I recall Chris White, who does carry a lot of stuff, used one on TCR 2016 and was keen on it. But he has since moved on to bigger bags.
I have Arkel's seat pack, which is also good. Has an internal frame which makes it rock solid, so much that I typically carry a litre bottle of water strapped on top, and fully waterproof, which is nice to have for me.