in the last 20 years I've used most everything that has come along. there are a lot of ups and downs for bags. I've been a physician assistant for just shy of 15 years, was a medic for several before that, and a firefighter the entire time. I've been lucky enough to evaluate a lot of products out there. I haven't found any one "perfect", but there's a lot of good amongst the not-so-good, and it really means figuring out your needs prior to purchase (vehicle only, shoulder bag, backpack, need refrigeration, do you need it to last forever or just evaluation period, etc etc).
My favorites were the Pacific Emergency Products ALS bags. Sadly PEP quit manufacturing bags in favor of ballistic sheets and body armor. You may find them out there on a deal, and I highly recommend them. Take a look on ebay there may still be some floating around. Easy to use/organize and built to last.
I've used Thomas Packs. Built like a tank, essentially waterproof, poorly designed, and VERY uncomfortable if you have to hump them very far. Can be overpacked quite easily, I've seen idiots pack them to over 60# of inappropriate gear. They will last long enough for you to hate them then pass them on for your children to do the same.
Conterra bags are typically built well, but I haven't grooved on their organization necessarily, a couple of bags have seemed a little wonky - YMMV.
Ferno's bags are pretty good, as are Iron Duck's. Not been thrilled with the zipper function on alot of the Iron Ducks I've used. Overall good bang for the buck. Not a big fan of the slick-sided ones that Iron Duck makes, they tear requiring replacement under OSHA (if you're using them for work).
I've had one StatPack item worth a damn, and it's their drug module (clamshell style, see through top, drug lock on the zipper). Most everything else I've seen/used is lightweight crap worth about one trip. YMMV.
Blackhawk makes top of the line gear in general, but I haven't had a chance to see much of their stuff (I have a bit of web gear and a couple of slings from them, impressed with quality....but you pay for it).
The new ALS backpack from 5.11 (model 84 I think) looks pretty good, but I want to see it in person to get a feel for quality and actual usability. It's probably going to be my next bag.
The Gall's brand bags are generally, IMO, junk. or at least were a few years ago.
Most commercially available prepacked first aid kits dont give you what you need, I concur wiht SunTzu's Nephew on this.
Dont be afraid of carrying two bags - can help weight distribution if you have to hump them somewhere, and can help with overall convenience of packing too. Depending on how their service is set up, it's not uncommon for medics to have three bags - trauma, medical/drugs, airway. Stay flexible.
Lots of my work has been in rather austere, isolated environments - somewhat like the trips we take. My experience is that trauma is a factor far less than medical problems - you're not as likely to hack off your leg as you are to develop chest pain, pneumonia, pyelonephritis, etc. This is where it starts to get tricky, you can get over your head pretty quickly due to a knowledge gap even if you have the right tools. Most kits, especially the prepack ones, are pointed right at trauma, and have little medical capacity in them. medical equipment and knowledge should be as important as food and water, but it is limited by realistic time commitment and money. Learn all you can, train to that level regularly, and Be Prepared. Consider a SPOT too....cheap insurance if something bad happens.