I've been following this thread for a long time and even posted a couple of times as I've been pondering a Billingham bag for some time. While I love the craftsmanship, materials and quality of Billingham, today I had a chance to look at both a Hadley Pro and an Ona Brixton at a local camera shop and I ended up ordering a Brixton when I got home.
While the bags are nearly the same size, the Ona is ever so slightly wider and deeper which just suits my gear that much better. In spite of this, the Brixton actually looks slimmer and tighter (not so baggy). I also like the flap fastening system better on the Brixton. The Hadley grab handle is more robust but the Brixton also has a grab handle which will work just fine for occasionally grabbing the bag. I will primarily be using the shoulder strap so the fact it had a grab handle is enough. I also like the looks of the Brixton better (black waxed canvas with reddish brown leather accents) it just looks more masculine/weathered and the look suits me better... I just can't help but seeing a man purse when I look at the Hadley. Unfortunately the other more masculine Billingham lines are way too big or have other issues. Billingham just doesn't make the perfect bag for me.
Anyway, the reason I post this is not to bash Billingham... as I said off the top, I love the craftsmanship and everything the brand represents, but it's just not for me. So if you're in the same boat, check out Ona Bags, they have some very high quality good looking products as well.