I have a ZPacks Multipack. It's very well made I think, stronger, less fragile than I expected. It's not a whole front pack system like an Aarn bodypack but capacity is really very generous for a pouch. For me it is best used for the things/tools that you need to get to more often on your trek like compass, windshirt, map, energy bars, dried fruit, camera, etc. And I thought I would love that set-up.
Thing is, I tried it and for the way I trek that system doesn't work very well. No fault of the ZPack Multipack, I think it's more inherent to the set-up in combination with my usage. I do mostly faster treks, with quite a lot of elevation changes, scrambling etc. If I loosen all the straps, breathing and temperature are ok but with the added weight in there the pack bounces too much for my comfort.
When I tighten the straps more to mitigate bounce, it makes me run hotter than normal and I sweat more. Since I need to ventilate well because of the distances/speed I like to do it doesn't work that well. I'm sure if I would go on more general speed hikes with other people that I would use it again and love it.. Again, the quality is all you could wish for.
So to answer the original question, it either makes me run hot, or it bounces (not both at the same time). I'm looking at adding snacks or some extras I want to get to on a side grab pocket and extra shoulder strap pockets if possible (also done by ZPacks). Not as generous space wise, but better for my usage.