Personally I find a backpack style bag to be a PITA to work out of for general use. Unless you are planning on carrying it longer distances, a regular style bag is easier to locate stuff in and get stuff out of without spilling stuff. I've used Iron Duck bags with two different EMS services and liked them; they're durable and they have an excellent warranty. I also like using a Pelican case for vehicle use in that it packs well and protects your supplies, but I use a smaller one than their 1500 series. Adventure Medical Kits makes decent, well thought out ready to buy kits if you don't want to be bothered putting one together yourself and if you've got the money.
Also having a bunch of smaller packs (kits) inside your main bag is great, but you can do the same thing with ziplock bags or inexpensive nylon bags if you're on a budget.
I'm also of the opinion that most folks way overpack on medical supplies, but that's just me. I can't tell you how many people I've come across who have stuff they aren't trained to use who say they carry it in case someone else may be trained to use it or for use only in a dire emergency.
I think that people tend to overpack, but of the wrong things, and tend to drastically underpack on certain items (like 4x4's). Part of the problem is that there are lots of items that are sort of single-purpose (eye patches, for example)....yes, they're nice to have, but you know what? A couple of 4x4's does the same thing. OB kits? I already carry everything I need, why duplicate it.
Commercially made kits are famous for this, and for loading the kit up with items that aren't really usable at all - scissors that can't even cut air, tweezers that are useful only for picking up boulders, etc. Then they don't include really important things like hemostatic agents (Quikclot gauze, for example). Modularity can be nice, but it can also be a hindrance - the Thomas series of bags, IMHO, force you to carry stuff their way, even if you don't want to.
I use a lot of ziplock bags (or ziplocks inside of vacuum sealed bags, to close up after I rip open the vac bag), as well. They're cheap, visible, lightweight, and sufficiently waterproof to protect most everything I carry - except for things like drugs and the pulseox, blood glucose meter, digital thermometers, which go in otterboxes for protection (the drugs stay in my refrigerator most of the time, too).
Too bulky/heavy. Need a soft bag for that. 
