I purchased this to replace the exact same model I purchased 2 years ago because the zipper on the original one broke (that is why this is 4 stars instead of 5). It fits well behind the headset. It does not get in the way at all. Installation and removal is very fast and it sits securely.
I have the medium size. It holds plenty off gel and chews for a long ride along with a small bottle of hand sanitizer and some wipes in case of mechanical repairs. You could put a multitool, inner tube, tire levers in there instead but I use a saddle bag for those. The hole for a charger cable is a nice feature but I have only used it once (it worked well).
It is waterproof enough (except for a long time in a severe downpour).
It is a very good bag for the price.