Everyone needs a minimum of 2 bags in their life. A 20-30L backpack, and a carry-on legal duffel. The backpack is up to you, but this should be your duffel. I've had mine for about a year, and can honestly say it's the best overall carry-on design you can buy. Most people who travel have realized you can fit more stuff in a duffel vs a similarly sized roller because you don't have to account for the wheels or handle. However, what separates this from the rest of the duffel world are the pockets, carry options, and clam-shell opening. Packing cubes take this to another level. Go with the Eagle Creek Pack-It Specters to save weight.
1. The tiny exterior pocket will hold a pair of shoes or a couple pairs of flip-flops, keeping them separate from your clothes. Best option is wear the shoes, pack the flops.
2. Small exterior pocket has organization for all your cords, wallet, passport, travel docs, etc. I always carry my phone, but found the padded pockets holds my Anker battery perfectly for emergency juice. Having this stuff accessible while not showing off the rest of your stuff is key.
3. Large interior pocket will hold a variety of cubes. I recently returned from 8 days in Costa Rica and brought 2 large cubes, 2 medium cubes, 1 small cube, and a toiletry bag with room to spare.
4. Two small interior pockets hold small items. I used one for socks, and the other for some emergency meds and a small travel camera.
5. Behind the small interior pockets is another that's great for keeping items flat. It held my suit and wife's dress for a wedding we went to last summer without any trouble. Alternatively, you can keep this pocket empty and use it for dirty/clean clothes depending on where you are on your trip.
6. Laptop sleeve. Want your laptop, but don't want to carry a second bag to keep it protected? Done.
7. Tiny pocket with zippered bottom for sliding over a roller. I don't use a roller, but it's good for holding my kindle and pass while boarding.
1. Two handles make grabbing it out of a trunk or overhead bin easy.
2. Backpack straps are wonderful if this is only bag you bring. Otherwise, tuck them away, and the use the...
3. Shoulder hardness for when you're also carrying that 20-30L backpack. You can remove this and stuff it into the backpack strap area when not in use.
Having the bag split in half makes packing and accessing your stuff much easier than a traditional duffel. You'll come to love it when you can just open the bag on the floor in the hotel, and see everything nicely laid out.
The Black Hole series gets all the recognition, but this is the better travel bag. Like everyone has mentioned, the 3-5 day suggestion is crap, and will easily get you through twice that. If you want to go full travel-pro, get one of those super compressible day packs (Sea To Summit, Osprey Ultralight, etc.) for all that Costa Rican coffee you're going to bring home...