I'm new to the Tom Bihn thing, but what I look for is modularity, made in America, lifetime warranties, and ruggedness. Tom Bihn has checked those all off and I purchased a Cadet some short time ago. I've been using it and I love it, but what I found was:
1) I carry too much stuff. I keep a first aid kit, sun glasses, a journal/notebook for whatever, a few pens, medications, contact lense case + fluid, some OTC stuff, and a momento or two from my recently deceased grandfather (more like a father, but I digress) along with a cache. It causes my bag to bulge and while it all JUST fits, it's not ideal.
2) I like supporting companies and buying more stuff does that.
3) With the amount of things I carry the side-bag style just isn't cutting it. A backpack is more ideal.
4) I may end up carrying more stuff as time goes on. It's just who I am as a person, I'm not a minimalist EDCer - I like to feel prepared to handle any and all issues tossed at me.
5) I can still make use of my Cadet when traveling. I bought an extra cache so that I can seamlessly pick up this bag and use it for those quick trips/as a personal item.
I searched and searched for a backpack that I wanted. I looked at every single competitor that fit most or all of the requirements/desires that I had. The next closest (for me) was Red Oxx. I do own a backpack from them and liked it somewhat, but it was just...it wasn't Tom Bihn. So I went with the brain bag. The *ONLY* thing I wished my Brain Bag had was the internal fabric that could be offered with the Halycon (if I'm spelling that right). My cadet has black ballistic outside and this wonderful blue checkered (white lined) material inside. I LOVE that material. It feels so good and works so well and adds this wonderful contrast. If Tom Bihn offered that....I'd ask to do a trade haha.