As good as you'll find... It holds a deceptively large amount of small items. I have a topeak alien II, a cell phone, car key, tube patch, and small pump in mine. It is not pretty on the bike, but functional and secure. The best feature though is it's water resistance. I dumped my bike in a stream (rider error) and there was several minutes during which it was fully submerged. All of my items remained perfectly dry. It was worth every penny and then some.
Nice Bag:
This bag is constructed of sterdy materials. It is a good piece of accessory to have especially if you do a lot of riding. There is more than enough room in the main compartment to fit your most important stuff. It did not fit behind my oversized seat, but I was able to utilize it elsewhere on my bike.
Good size and durable:
Size is just big enough for my morning rides (ID, keys, patch kit, etc). Mine had rubber strap for seat post, it broke on first ride. Replaced with a big rubber O-ring from auto parts store, no issues since. Overall a great bag.
Nice Product:
I'm so glad I took the time to read previous reviews about this bag. Unlike competitive bags that fall off due to ineffective velcro straps this bag is secure and well made. The straps go throught the seat rails and buckle underneath. It is the perfect size and seems extremely well made. A great value!