I use this bag on a Surly Midnight Special for my road rides.
The bag is 7L, just big enough for a tool kit, spare tube, lightweight jacket or vest, spare glasses, gloves.
If you carry a lot of gear, its not going to be big enough; but for my day rides its perfectly sized.
It has a nice automatic one-way valve so its easy to squeeze the air of it.
The bag has some high visibility yellow strips and you can mount a taillight on it as well.
I have had this bag on the bike for about 2 months and I like it very much.
I also have an Apidura frame bag on another bike...they make a quality product IMHO.