Mark Traverswrote a review of Timbuk2 Seat Pack XT on June 18, 2015
Familiarity: I've used it several times
I got a size small for my road bike and I'm really happy with it. Unlike most other saddle bags, there's no Velcro on this one. There's a bit of shock cord and a small clip that wraps around your seat post and a bit of nylon strap with two buckles that wrap around he seat rails. I'm pretty stoked that there isn't any velcro to worry about he ends/corners creeping out and scratching up my legs. The shape of the bag is longer and narrower than most other ones I looked at. It can be a little weird depending on what you're trying to cram in there but it does a really good job of staying right under your seat and not sticking out beyond the saddle at all.
The size small is great for my road bike - fits one tube, C02, and an inflator head. There's (barely) space to shove a key in as well. If you're looking to carry your phone or use this on a mountain bike I'd suggest sizing up. I've had this for about a month so can't say too much for long term durability, but it seems like it is built well and should be able to hold up just fine