Meh. It looks good, it secures easily to the bike, and it has what seems to be a high-quality zipper. However, even the large size isn't really very large. The dimensions shown are the MAXIMUM dimensions of the most-sticky-out parts, so the cavity is actually nowhere near the full size the dimensions might suggest. (On the large, the 8" dimension is from the deepest point at the apex to the very tip of the pointy back. Since the thing is wedge shaped, you really can't fit an 8" x 3" x3" object inside, for example.) I have doubts that the usable volume is actually 1.2 L. I think if you were to put the bag into the smallest possible cardboard box that it would fit inside, that box would have a volume of 1.2 L.)

Anyway, I've packed the following items into my large bag, and it's a very tight fit with absolutely no room left over for a wallet or phone (although I can still fit a house key, a few notes, and an ID card): 2 tubes, 1 multitool, 1 patch kit, 1 CO2 cartridge, 2 vinyl gloves, 1 small rag.

Otherwise, there are two small flaws I can see so far. First, the bag is not water-proof. (I put a small, open plastic bag inside before loading it, so that shouldn't be a problem.) Second, the clip for the taillight is angled downwards quite a bit. (I mounted my taillight to the very top of a seat-stay instead, under the bag.)

Very stylish looking bag, has a nice zipper does not rattle around and at a very good price. ðŸ‘ðŸ‘ðŸ‘ðŸ‘ðŸ‘
Dimensions: 8.0 in L x 4.2 in W x 4.0 in H. Capacity: 1.2 L. Weight: 4.8 oz.
I am using the large BV Bicycle Y-Series bag on a 1995 Trek 750 hybrid bike with a Serfas Full Spring Suspension saddle.
If you need a larger bag take a look at this one it is expandable to 1.5 L and has a key clip on the inside of the lid
BV Bicycle Strap-On Saddle/Seat Bag, Large, Black

The rear light mount is angled downward at a angle a little to much and you have to have a good bright light to be seen well,
I am using a Planet Bike Grateful Red Bike Tail Light, Red/Black

Iphone 6 plus and or 7 plus will fit, as well as the Samsung note 3 or 5, S7 edge or if you're like me your 6 inch by 3 inch prepaid Tracfone will fit.
I am using the large BV bicycle bag on a 1995 Trek 750 Hybrid with a Serfas Full Suspension Hybrid Bicycle Saddle.
More information in my comment section.

Thanks for taking the time to read my review...
Campjeep

I got the small size and it barely fits my iphone 6 when it's in an otter case and a set of keys.... which is all I had wanted to put in there anyway. If you are thinking of storing anything in addition to a phone and keys then you'll need a larger one, also if you have the plus size iphone or samsung note you will need a bigger size, the problem with that is you'll need to measure and be sure there is enough clearance between the seat and the rear wheel.

This item is as expected. It fit on my bike well and attached to the seat pretty easily. It also worked well with the Blitzu USB led lights I purchased. I was able to fit 2 tubes, tube removal tool, utility tool, 3 CO2 cartridges and the CO2 inflator in the Large strap on bag option I purchased. I have not rode with it yet but will be tomorrow.

I got the Large size (after reading other reviews). It's plenty large (it actually extends beyond my Trek Domane's seat by about 1.5". It's a good size all and all. Build quality is good to very good. The door is zippered with a nice zipper and has a mesh/net pounch (for a few folded up $20 bills, etc). Some wrote a review that claims the flap opens upward (it DOES NOT) and therefore dumps everything on the ground. That person mounted the bag UPSIDE DOWN. It mounts like every other seat bag on planet Earth (the exact opposite as the photos from that review imply). My is mounted well and doesn't make any noise or do anything that any other seat bag I've had ever did. I'm very happy with my purchase!