Hi, I use both the fixing and the Mondo pack, I have found no problems with the fixings and I have them fitted to all my saddles, despite reading in the past that riders have lost their packs, but I'm always checking constantly that it has not fallen off especcially if I hit some bumps. The only problem I do find with the fixing is that I have to remove it if I have to fit my Carradice support for longer rides, which is why I'm beggining to think, velcro is more versatile for attaching bags to saddles, plus I don't have to buy all these fixing brackets.
The Mondo bag is good, I have the one without the water bottle and is very secure especcially with the two velcro straps which go around the seat post and no movement at all, but you do need enough seat post exposed to really fit one. As for packing it, I use mine for long rides and carry lights, food, charger for Garmin, repair kit, inner tubes, lock etc ie lots of small loose itemsand I find it can be a pain at times that with the side opening zip, things start falling out when you open it, and then when you try to zip it up, you have to manipulate items upwards again as everything has fallen down. In reality it would be better to lay the bike on its side when you open it up, in the same way that you pack it before mounting to the bike
But if I did a long ride and the weather could be inclement, along with all the stuff I have mentioned above and trying to include some extra clothing, there would not be the space, but on a shortish ride, lots of room for that extra clothing, ie small waterproof jacket, arm and leg warmers along with repair kit and a few bits and pieces.
Also there is two sides too it, but only a thin cloth barrier between both sides, the smaller side, only really holds a small repair kit, one innertube, two at a squeeze, but put two much in and it bulges through into the larger compartment, reducing its capacity