X-pac material seems good so far. There is a tiny bit of leakage through the zips during a big downpour. The material at the bottom of the bag I don't believe is waterproof which makes no sense as 100% coverage of the rest of the bag is. As a cyclist this isn't good as you get a lot of spray from the rear wheel hitting this area. I would appreciate some form of confirmation of this from Boundary though.

The shoulder straps a very stiff and wayyy too rough texture, they hurt my neck without moving my t-shirt to cover them. I am going to look at getting some cotton strips sewn on the help prevent this. It's not a HUGE deal breaker but it is the main reason for not giving 5 stars.

Love the looks of the bag and the main compartment Fidlock buckles are a joy to use. Also the build quality seems quite good so far, a small bit of fraying here and there (after only 2 weeks) but I don't think I've ever owned a sewn garment that doesn't do this (it's not falling to bits, just tiny bits of thread poking out).

The back ventilation is quite impressive, the channel along the spine works really well.

I'd probably make the pads slightly thicker to exaggerate this effect. And possible add some horizontal channels too.

The buckle on the chest strap isn't very ergonomic. It's quite hard to open with 1 hand. Fidlock do offer 1 handed buckles similar to this which you press together and they open (Fox Racing Helmets use them). I would say the upgradeable chest strap is probably worth…