I use this on my commuter bike to hold glasses, keys, and phone. I got the extended size because the other smaller one does not hold that much. This is long enough for a glasses case. I've used it in the rain and it kept stuff dry. The zipper is self-sealing. Perhaps in a really long rain ride things might get moist, but you can always use a ziplock inside.
This top tube bag looks great and works great. I love being able to eat snacks by just unzipping the bag and reaching down. That being said, after just 2 rides the fabric that is against the stem has started to fray, which makes me question its durability. There should be an extra patch in this weak spot to help protect the bag like there is on the saddle bag. I ended up buying gear repair tape to put in this area to keep from further abrasions. Hope it helps.
Motivation for purchase improved during-the-ride gear accessibility (as opposed to under-the-seat saddle bag or stuff random in jersey pockets), target items granola bars, cell phone, maps, gps, cash, etc. I got the XL bag thinking it would be substantial in size, but I can say it is just enough in size to stuff a guy's wallet, a two-pack granola bar or two, and a cell phone in. I can't imagine the smaller version being very useful. Too small for a bike pump when both below-the-frame spots are allocated for water.
Pros does improve access for a small handful of items, but not as many as you think, zipper tab easy to grasp even with gloves on, nice styling, lightweight, aero-dynamic, nice material construction, not too overly large to affect step in (at least for me)
Cons not overly huge, as mentioned at BTD, not waterproof (no flap), by design ALWAYS lists to the right, enough to occasionally contact your right knee during turns, general riding
I may be able to add a foam block to beat the right list issue. The list is about 34 inch, regardless of wind. I have a very large tube and would imagine it would only be worse with a smaller tube.
I still like it, but the listing issue is reason for lowering the value rating, the designers should have gotten that right out-of-the-box.
This is the second one I've purchased. As I stated above, the top tube pack is light, well made, and stylish. It functions very well at the other end of the top tube fastened to the seat post. The only consideration to be aware of is if there is not much space between the headset and the stem you may have a hard time attaching the front strap as it is wide and positioned in the center.