I have a Galls mega medic bag that was given to me, I don't use it in the "field" but use it to store training gear. It's huge and seems well made. the dividers are a little bulky but very rigid, and it has a waterproof bottom. To give you an idea of size, you can put 3 x 1000ml iv bags & giving sets , plus a selection of cannulas etc in one of the side pockets.
I also have copy mega medic bag, that I got off a dealer on ebay (also free, as my order was late). there is a marked difference in quality, the Galls one is much better quality but probably 3 times the price.
I keep a Galls Mini Medic Bag in my car for use as a first aid kit. Even though it's small I can fit plenty of supplies in it and it doesn't take up much space. Also got that bag for free too !!