This bag has some nice features. ðŸ‘ðŸ‘ðŸ‘ðŸ‘ðŸ‘
I have the large, I like that it is expandable up to 6 inches high, has dual zippers with (with thumb tabs sewn into the bag at the bottom of each zipper, a key hook or clip and mesh pocket to secure your keys but I wish the mash pocket was a little deeper, nice 3M Scotchlite reflective stripes on both sides and rear, loop for light on rear.

I am using the large BV bicycle bag on a 1995 Trek 750 hybrid bike with a Serfas Full Spring Suspension saddle.
If you have a road bike with a very small saddle or any bike with your saddle set down low you might need to get the medium expandable size.
In my comment section I have all that I carry in the bag.

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.
This bag seems fine on longer group rides. Item Dimensions are 8 inch long x 4 inch wide at rear x 4.8 inches high at rear of bag and expandable to 6 inches high.

If you want a more stylish slightly better quality and a tad smaller in the large size bag for everyday rides check out the BV Bicycle Y-Series Strap-On Bike Saddle Bag / Bicycle Seat Pack Bag, Cycling Wedge with Multi-Size Options

More information in my comments.
Thanks for taking the time to read my review...
Campjeep
( Picture is of the medium)

As of only having used this bag once, my experience is limited, but my initial impressions are that this is an awesome little bag.

Pros - So right off the bat the first thing I noticed is that this is an extremely well made bag. The stitching is sturdy and robust, and it is obvious that whoever made this bag spent a lot of time making sure that it was crafted properly. I have no doubt that this bag will last me a couple years if properly taken care of. Another Pro of this bag is that it has 3M reflective tape on the side, now as somebody who uses their bike for commuting, being seen is always something that is of importance to me, especially at night. I have attached pictures that show how bright that tape is when hit with a bright light. And my last major take away from this bag is that it has tons of space inside. I have plenty of space to fit my full wallet, my phone, my keys, tools and a spare tire.

Cons - Really the only con, and this one is a bit of a stretch, is that the bag wiggles just a tiny bit when you're handling it. Now this is completely unnoticeable when you're riding the bike, but due to it only being attached at two areas it seems to move a little bit when you're opening and closing it. As I said, this one is a super stretch, I honestly couldn't think of anything that is a real negative about this bag.

In summary, if you're looking for a sturdy, well made, and well thought through bag to use on your bike whether you're a commuter, racer, or just somebody who wants some extra space, this bag is for you. I could not recommended enough.

I will update this review if anything arises in the next couple months.

This hangs below the seat on my Trek carbon fiber road bike.
It's a size Large. If I had to I could cram a light windbreaker into it along with my tube and tools.

Pros:
"¢ It's just the right size for a spare tube, some tools, etc.
"¢ Build quality is excellent (especially for the cost; it's a serious bargain)
"¢ Strapping system is secure"”it doesn't wobble
"¢ Strap on the back holds a standard rear light (which I use strobing in the daytime on roads)
"¢ Rear opening is easy and well positioned
"¢ Key retainer clip is strap-attached inside.

Cons:
None.

I'm not surprised that this cycle seat bag has such high reviews (and top seller in category if I'm not mistaken). The bag comes in three sizes, for this review I will only discuss the Large size.

Good:
-Construction is solid. No obvious defects or points of potential failure.
-Ample room. I was able to fit (without using expansion):
-Multitool
-CO2 cartridge and adapter
-spare tube
-two tire levers
-latex gloves with patch kit and link
-2 cliff bars
-a concealed carry (Sig Sauer P238).
-There was space for my car key and my phone as well, but that would be about the limit without expanding it.
-Internal hook and mesh netting for keys. Extra layer of security.
-Simple attachment mechanism.
-Thin profile -- does not stick out wider than even a narrow seat.

Bad:
-Seat tube strap is FAR too long to give a tight fit. You will have to cut and resew if you wish to have it snugly attached to seat post.
-Has a horizontal strap on back to attach light. Most lights attach horizontally because they would normally go around the seat tube. Consequently, I had to attach my light sideways. An additional vertical strap just above the horizontal strap would resolve this problem.
-Bag does sway side to side, but this is unavoidable. During riding I do not think it will affect anything. Only way to avoid is to have an internal frame so you can tighten the seat strap completely. Without frame, the bag will just collapse in on itself. During rides, it may be a good idea to cinch down the strap fully and let the bag compress onto the contents to minimize swaying.

Bottom line: The bag is great and I would absolutely recommend it. To the manufacturer, I would suggest incorporating my two recommendations (seat tube strap length & vertical strap in back) for future iterations of this great product. Until the manufacturer applies the recommendations, a rider can always take the bag to a sew shop and have the modifications done for just a few dollars.

I've included a couple pictures: A side profile with ruler (~6.5" deep) and a perspective angle.

Happy with the medium size. Lightly structured but not too stiff. Easily holds spare tube, two levers, patch kit, and multi-tool with a bit of room to spare. And that's BEFORE you unzip the extender. Tucks nicely under the saddle and doesn't stick out the back. One strap velcroes around the seat post, one strap buckles under the saddle rails. Simple to attach and remove. Materials and quality seem pretty sturdy without being too heavy.