I bought a large duffle middle of last year to replace an old Da Kine Transporter duffle I got as a demo model some 7 years ago. I work in Afghanistan and Iraq for USAID and travel to a lot of different provinces via military vehicle convoy, helicopter, fixed wing flights, and SUVs and stay on bases in anything from a 130 man tent to single room. I wanted something that could replace the Da Kine Transporter which was just a massive vessel in which to through anything and everything I might need to cover every eventuality and could be thrown in the back of an SUV or Humvee and not get destroyed and at the same time make it through the Heathrow baggage handlers and aircraft hold without being ripped open with my stuff being spewed everywhere.
When the Large TNF Duffle arrived I thought it was tiny and almost sent it back for the XL. I guess my Da Kine Transporter was 140 odd litres or so. But I ended up keeping the Large TNF Duffle and love it and would totally recommend the Large. I tend to stuff a bag with what ever I have until it is full instead of choosing what to take carefully whether it fills the bag or not. I realise now that I would often overfill the old 140L bag with crap I didn't need resulting in a bag that was awkward to carry and often incur excess baggage charges on commercial air flights. The Large TNF Duffle will hold enough stuff for a good week or two in the field (with size 13 feet a pair of boots can often take up an entire smaller bag). The compression straps on the side are great if the load is smaller (in fact I did all my shopping on my last trip home using this bag, started with it compressed and toured the shops filling it and expanding it as I went). Material is bomber, very tough and durable, mines been dragged through mud, thrown into and out of planes, trains, and automobiles and retains it's integrity.
I suspect the XL would be good for long haul trips to get a good amount of gear from A to B, but to go from A to B to Z to X to the middle of F nowhere the Large would seem to rule supreme. The internal mesh pocket under the lid is great for all the small bits and bobs that you might want quick access to. The dual zipper pieces are ideal for padlocking so I padlock the bag shut when in the 130man tents and what not. Large main bag interior is accessed by the big top opening which allows good access to the full interior compartment. Rucksack straps are comfy and allow for prolonged carrying of the pack, and the end handles have so far held up to being yanked at to withdraw the bag from a mass pile. Even the name / contact details window is great, I just put a few business cards in there and can easily identify it and have business cards to hand when needed.
All round great bag, Medium I guess is small and ideal for Airplane carry on Luggage, Large is a great all round travel size for multi point short to medium trips, and the XL would be great for transporting gear from point A to B and longer trips.