This is a very lightweight, portable option to a wheel aboard carry on.  I wish it was an inch or two thicker to be closer to the max carry on size but it serves my purposes well.  The first time I used this was on a 12 day trip to Europe where we were taking trains from city to city and I didn't want to be dragging my luggage on the subway.  This was my only bag in addition to a small shoulder bag.  I fit 6 days worth of clothes in here plus my toiletries and a few miscellaneous items.

The bottom pocket is great for a spare pair of shoes or dirty laundry since it has a flexible divider.  The top pocket is great for your 3-1-1 liquids bag for quick access going through security.  The small front pocket is great for stashing boarding passes or a thin guide book or a stocking cap and gloves.  The back pocket which is designed for a laptop is also great for all your important paperwork for your trip since if you wear this as a backpack that side is inaccessible to pickpockets.  The mesh pocket in the flap when you open the main compartment is good for small items like socks and underwear.  The straps in the main compartment help keep everything from sliding around when cinched tight.

I have only carried this as a backpack and my one suggestion is to add a sternum strap for comfort which you can buy on Amazon or elsewhere from the manufacturer.  Using a packing folder organizer in it helps add structure but isn't necessary.  I removed the shoulder strap for carrying it as a messenger bag and use that on a different bag.  I have used this on a half dozen trips and its great for anywhere from a weekend to a week if you're a light packer.  I even fit it under the seat on a smaller plane that didn't have dividers between the underseat area in front of my husband and I.  The small size would probably be better for that purpose but I don't really need two of these.

For trips where a roller bag is inconvenient or too big, this is my go to bag now.