Douglas N.wrote a review of Timbuk2 Seat Pack XT on October 5, 2015
Familiarity: I've put it through the wringer
Good: No velcro, good quality. The rear of the bag is a deflector but isn't sewn down on all 4 sides so I clip a tail light in there.
Bad: The small (which is what I got) is really small so it doesn't fit much in there. Other reviewers will tell you what they fit. I stick a garage opener (no clip) and a key. Not really worried about flats on the roads I frequent.
I'm not returning the bag because I do like it but if I had to do it again, I'd get a medium.