I have used this particular piece of equipment before and I would like to add, for the sake of the conversation, some minor points that I ran into during my experiences with it.
Overall I think the design is excellent as a one stop shop for absolutely necessary equipment in the field (poncho, mre, ww top, eyepro, earpro, gloves, weapon cleaning kit, etc.). The removable beavertail was very convenient if I felt like strapping my ACH to the back if the situation permitted, and I usually did that if I was able to because who the hell likes to wear their helmet if they don't have to. I also pushed it into the ammo bag role on a gun team and it was just the right size to carry 300 rounds of 7.62 link in useable configuration. I would just drop pack when the gun was in position, unzip, link up, and prepare to feed rounds. In the situations where the beaver-tail wasn't necessary, being able to take it off shaves a little weight and unnecessary bulk off of the overall profile of the pack. I, like everyone else who has to, hate carrying unnecessary shit. The price is also right on this one when you compare it to its Eagle/First-Spear/Tyr brethren.
Some negatives that I found in its use were that under reasonable load the little ovaloid rings at the top of the detachable straps would shift into a vertical position. Not a huge issue, but annoying. It could be fixed by incorporating some form of hard sewn strap retention into the body of the pack, going out the top, but still being removable. Another issue with the straps was that after several months of moderate use the shoulder strap stitching where the straps attach to the bag began to pull out, and I was worried for a bit one, or both were going to bust and (pun alert) leave me hanging. The third issue I found after some rough and tumble with the pack was that the friction between the unfinished edges of the plastic frame-sheet and the 500d pack exterior was rubbing a hole in the pack on the back. Also, I found that the thing would ride exceptionally low when using it mounted to a plate carrier. I would have to mount the molle interface three rows higher than one would expect to alleviate this issue. This could be fixed by widening the profile of the pack by two molle rows, and shortening it by a few inches.
That's just my two cents on the kit.